I can speak for the history of WWII, but boy did it ever.
The historiography of WWII has had some serious issues over the years. The official history of the war written by the Americans was informed by none other than former OKH chief, Franz Halder. The 1950 combat infantry manual for the US Army that talked about Soviet fighting capabilities was HEAVILY informed by the Germans... and so was popular history. Halder has his diary, and Heinz Guderian and Erich von Manstein wrote memoirs that were widely published in the West by the 1950's.
The large histories of the 1960's and 1970's suffered from having access to German memoirs and remembrances, but not always their sources if they were captured by the Soviets or destroyed. Soviet sources were hidden and Soviet writings and memoirs had some serious issues:
As such, there are issues that plague histories that rely on the Germans and sparsely on the Soviets. Here are just a few examples.
Second Kharkov and the Donbas Campaign. Reading Alan Clark's Barbarossa (first published in 1965), it seems like the Soviets learned in the Spring/Summer 1942 how to retreat and withdraw before being encircled. There is now serious doubt that this was the case. Internal documents (Glantz and the first book in the Stalingrad trilogy has a lot on this) shows that these withdrawals were often piecemeal and against orders from Stavka (Soviet High Command). Veteran testimony that has come out recently sure talks about morale issues and retreats and a shattered army. This is all new.
We now have documents showing that Chuikov tried to move the HQ of the 62nd Army to the East bank of the Volga... twice. This is laid out in a book that is really high on Chuikov (Michael K. Jones - Stalingrad: How the Red Army Triumphed).
Speaking of Stalingrad. What a good example of how Stalin and the Red Army learned how to attack at one place at one time. Right? I mean, we have internal cables from Zhukov telling officers how to fight in the Winter of 1941-1942 (he literally had to tell officers that they should use recon, then air, then artillery, then have more dudes than the enemy, check Glantz - When Titans Clashed, stuff like that which could be embarrassing to the Red Army is far more widely available now). Anyway, Operation Uranus wasn't the only attack. The Soviets had multiple operations around Rzhev that led to hundreds of thousands of casualties... each. These were, by and large, lost to history until Russian historians got access to the archives and the West got access to them.
The Purges are being reconsidered. I would read Richard Overy "Russia's War," but new numbers post archives show that the Red Army officer corps was larger than we thought and that more men were let out of prison and back into the army than we thought (think Rokossovsky).
Some historians think that Lend-Lease was FAR more important than previously thought. Jonathon House, using post-archive numbers puts the number of American trucks up to 500,000. He argues that Kursk would not be possible without US help, adding years to the length of the war. Albert Weeks "Russia's Lifesaver" brings some of those Russian numbers to an English translation if you want to check them out. I find the book not super useful, but the figures are nice.
You get chilling internal memos like the NKVD records from the Winter War against Finland. When they were considering how to re-write infantry manuals after losing tens of thousands of men, one commissar noted that they needed to start training men how to go to ground under fire (Catherine Merridale - Ivan's War), yeah... crazy stuff.
David Glantz is probably the go-to on this for a Western reader. His Stalingrad series uses Soviet and German records to painstakingly detail the day by day order of battle and location of the units as the battle unfolds. Reading that can show that they often disagreed on what units were where (like Stalingrad, Chuikov gave the Germans two divisions that weren't actually present).
I'm sure I'm leaving stuff out, it would be impossible not to, but this gives you an idea of some of the things going on in the historiography.
I recommend listening to dudes smarter than myself on the matter: (all of these lectures are available on Youtube)
-David Glantz - The Soviet-German War, 1941-1945: Myths and Realities
-Jonathon House - The Three Alibis
-Eastern Front - Final Victories (WW2HRT 31-06)
I recommend reading:
-Glantz and House - When Titans Clashed
-Richard Overy - Russia's War
-Stephen Fritz - Ostkrieg
This is only tangentially related to my area of expertise, so mods, feel free to remove my comments if they are not worthy.
Among the most famous cases where new information affected the perception of historians was the case of Alger Hiss, an American official accused of being a Soviet Spy shortly after World War II. He was indicted and ultimately convicted of perjury about his supposed spying and eventually served 3-4 years in prison.
The Hiss case was, in many ways, the absolute Rorschach test to determine a person's political leanings. Those on the left mostly had an absolutely belief that he had been railroaded during an era of anti-communist hysteria. Those on the right felt it was clear that he was a Soviet spy working on behalf of his foreign masters. This extended to historians as well. There seemed to be little middle ground for nuanced takes.
After the fall of the Soviet Union, the Hiss case was among the first to receive intense interest. Ultimately there was no clear "smoking gun" establishing Hiss' guilt, but there was more than enough evidence to make it clear in most historians' minds of Hiss' guilt.
A good rundown of the entire story can actually be found on the CIA website: https://www.cia.gov/library/center-for-the-study-of-intelligence/kent-csi/vol44no5/html/v44i5a01p.htm
In terms of my area of expertise (left-wing European terrorism of the 70s-80s) there was an absolutely shocking revelation after the fall of the Berlin Wall.
West Germany had been plagued by the Red Army Faction for almost 20 years; this Marxist terrorist org had bombed and murdered many Germans and Americans in West Germany in their effort to kickstart global Revolution in the west.
Members of the RAF were often extraordinarily successful and remaining hidden and underground. The West Germans knew that sometimes RAF members would go through East Germany on their way to places like the middle east; the East Germans knew who they were and tolerated them (typically grilling them for hours about their radical affiliations, before allowing them to continue).
What was unknown was that East Germany (or more accurately a small subset of the East German Stasi) had allowed 11 wanted members of the Red Army Faction to flee to East Germany, provide them with new identities, and live in East Germany as carefully hidden citizens. And even more shocking was that some members of the RAF who stayed underground in West Germany, traveled to East Germany to underground specialized weapon's training from the Stasi, before heading back to the west to use their new-found expertise (including an unsuccessful 1981 RPG attack in Heidelberg on a high ranking American General).
Though these were Stasi efforts, it has been alleged that the Soviets were either somewhat or completely aware of these efforts. Masha Gessen, a superb journalist, made a startling claim in her biography of Vladimir Putin that when he was stationed with the KGB in Dresden (mid to late 1980s), he regularly communicated with underground RAF members who were travelling through and would negotiate with them to have them bring him prized West German items like Blaupunkt stereos.
I have yet to actually confirm this story; and it does read a wee bit "perfect" (the current head of Russia just happened to be work with terrorists that were part of an ongoing war with the west). But I have no particular reason to doubt Gessen, who claimed to have learned it from an unnamed former RAF member she interviewed.
During the Cold War it was often assumed on the right that the Soviet Union was the master behind all forms of terrorism across the globe. On the left it was typically assumed that each group, be it the People's Front for the Liberation of Palestine General Command, or the IRA in Northern Ireland, or ETA in Spain, or the CCC in Belguim, or the Weather Underground in the US... each group was essentially independent and homegrown. It was also assumed that Eastern Governments generally did not try to support radical movements in the West; they were as much interested in detente and maintaining the status quo as the West.
The reality of the era, with knowledge gained since the fall of the wall, shows that the truth was more in the middle. The soviet union, and particularly East Germany, were not actively leading and directing the various global left-wing terror groups. But they sure did take an active and on-going support role.
This answer can change quite a bit depending on which historians you're talking about. I can't speak for Westerners, but the opening of the archives radically changed the burgeoning Soviet/Russian humanities.
Before we begin, it's important to note that the opening of the archives wasn't a sudden shocking event - it had several years of buildup. Gorbachev's policy of glasnost ("openness") shook the tiny circle of Russian social scientists to their core. For several decades the USSR had been funneling resources in academia towards what we would today consider STEM courses, at the expense of social sciences. Additionally, strict censorship laws prevented too much intellectual exploration. If you wanted to get big into academia in the Soviet Union, then you would aspire to be what was known as a "fortochka". A fortochka is literally a type of window found almost exclusively in Eastern Europe, that is designed to let air into a room without compromising the room's temperature. This is obviously incredibly useful in a stuffy room in the middle of a blistering Russian winter. However, certain social scientists and academics became known as intellectual fortochkas, in that they criticized the government and got away with it, because the criticisms they would make wouldn't be radical enough to cause any serious change. They could bring in new ideas and theories without compromising the Soviet system, much like an actual fortochka lets in cool air without compromising the indoor climate.
Gorbachev's glasnost resulted in a boom in the social sciences, as now there was more room for more dissenting opinions and ideas. Fortochkas began popping up all over, and as they did many began to make a startling discovery: the Soviet Union was woefully behind the rest of the world in terms of the humanities. Psychology had basically not progressed since Freud, and many new areas of studies simply did not exist (eg. Women's Studies was virtually, if not literally, nonexistent in Soviet universities). Accompanying these discoveries was a sense of national shame and humiliation. Ever since early schooling many Russian children were implicitly taught that Russia was first among equals within the Soviet Union in particular and the world in general. From a young age many were taught about the great successes had by the USSR during the Great Patriotic War (WW2) and how the Soviet Union was the most powerful nation on earth. The policy of glasnost shattered these illusions. A poll conducted in the late 1990s in Russia showed how the average Russian, despite being materially better off than they were ten years prior, felt they had it worse. The poll conductors chalked this up to Russians now comparing themselves to the rest of the world.
Compounding this was the release of many previously classified documents involving and sometimes authored by Stalin. The vast majority were simple, boring, administrative papers and memos. However, several provided what was at the time a shocking revelation in Russia: the extent of the gulag system and executions. What was new wasn't so much the information - the gulags and executions were hardly a secret - but the scale and perceived heartlessness of Stalin. Long lists of names put forth for execution with simple ticks next to them appalled many of the Russian intellegentsia. Former party officials who released the documents and conducted investigations were said to be shocked and deeply saddened, as not even many high-ranking party members were privy to this information beforehand.
But getting back to your original question, I wouldn't say the growing transparency or declassification of archives radically changed perceptions of previous events, but it did create a sense of shame and fear of being left behind within Russian academia and intellegentsia.
Source: The Future is History: How Totalitarianism Reclaimed Russia by Masha Gessen
This has had a profound effect in terms of studying nationalities within the Soviet Union.
Before the archives were made available, there was no way to accurately analyse the actions of the Soviets in terms of the nationalities within the USSR. And considering there were over 100 distinct nationalities (the number changed throughout the lifespan of the Soviet Union as groups were merged or formed, depending on the politics of the time), this was important.
For years the dominant theory was that the Soviets were "breakers of nations" (to use Robert Conquest's phrase from his 1992 book, Stalin: Breaker of Nations). They would go into a region and consolidate power around the ethnic Russians, forcing down the local ethnic groups and destroying whatever national identity they had. This also had the benefit of working in tandem with Marxist ideology, which abhorred nationalism: it was seen as a bourgeois means to create divisions within the working classes so they would fight each other rather than the capitalists. The seminal work on this by far is Richard Pipes' The Formation of the Soviet Union: Communism and Nationalism, 1917–1923, first published in 1954 (and subsequently updated in 1964 and I think another time). He was devoted to this thesis, and his book follows that idea, that the Bolsheviks came to power via putting Russians in power.
It took the fall of the Soviet Union in 1991 and the subsequent opening the archives to bring about a proper shift in how scholars interpreted Soviet nationality policy. While some had noted that there was more to the USSR than just the Russians, a view presented by Alexander J. Moytl as early as 1989 in his “‘Sovietology in One Country’ or Comparative Nationality Studies?,” it would not be until a few years later that more in depth scholarship emerged. In this regard there are two publications that set a new course on how to analyse the subject. Ronald Grigor Suny’s 1993 book The Revenge of the Past: Nationalism, Revolution, and the Collapse of the Soviet Union was one of the first treatments on the subject, and it would have an impact on the understanding of the subject as one of the first to argue that nationalism was promoted, not stifled, within the Soviet Union. Yuri Slezkine’s "The USSR as a Communal Apartment or How a Socialist State Promoted Ethnic Particularism," published in 1994, has also had a massive impact on the way scholars interpret Soviet nationality policy. Jeremy Smith’s The Bolsheviks and the National Question, 1917–1923, from 1999, further expanded on this idea.
Since 2000 two major works have been published, that while differing in their interpretation, have set the standard for a modern understanding of the Soviet Union’s early policy towards nationalism and ethnic identity. These would be The Affirmative Action Empire: Nation and Nationalism in the Soviet Union, 1923 – 1939 by Terry Martin (2001) and Francine Hirsch’s Empire of Nations: Ethnographic Knowledge and the Making of the Soviet Union (2005). Martin argues that the early Soviet policies, korenizatsiia, were designed to promote the minority ethnic groups in the USSR, a Soviet version of "affirmative action" as he said. He then states that this policy was reversed when it became apparent, to the leadership at least, that it was not having the desired effect, and thus the Stalinist repressions occurred. Hirsch’s book is slightly different, in that she approaches the way the Soviet authorities categorised the various nationalities in the state and how that information was both formulated and then utilised for policy-making. Martin's book in particular is a major conerstone in regards to Soviet nationality policy today, and you can't find any book or article on the topic that doesn't include it in the bibliography.
This has thus led to a change in understanding of how things worked within the Soviet Union, especially the early years pre-Stalinism (c. 1936). Rather than repress nationalities, the Soviet's encouraged them in their nationalism: paradoxically it was seen as an important step, as the theory was they had to move from their "primitive" way of life to become nationally aware, in order to move past that stage and into proper socialism. This was not something that could be properly understood until the archives were opened and scholars were able to see the discussions of the various Party and government agencies responsible, and why they acted as they did.
For me, the most striking material was that which documented how Stalin worked with his "team". On subjects like collectivization, famine and the purges, you can now only read Robert Conquest to see "what people thought then" -- but you can't read it as contemporary scholarship, there's just too much information that he never saw.
Books like The Stalin-Kaganovich Correspondence, 1931–36 give insight into just how Stalin ran the Soviet Union; everything in this was previously classified and unavailable. Sheila Fitzpatrick's extraordinary On Stalin's Team: The Years of Living Dangerously in Soviet Politics draws heavily on on materials that were previously unavailable. Similarly Stephen Kotkin's biography of Stalin-- unthinkable without the opening of the archives; pretty much anything written on the subject before the archives were opened is now of purely historiographic interest.
It's not so much that this "changed" how we understand Stalin to have worked-- before these archives we just had no idea. There was a brief thaw with Krushchev where some details came out, but Krushchev had his own interests and until 1991 we have only an idea of "What the Soviet Union under Stalin _did_", but very little insight into just how things actually worked. Prior to 1991, we had almost no informal correspondence on political matters by anyone, just official statements . . .
It gives us insight into big things, like Stalin's personal involvement in the purges
For example, it indicates that Stalin himself was the chief architect of the infamous law of 7 August 1932 that threatened 'draconian' measures against people who stole socialist property, and which was used during the famine to punish people who stole grains of corn (pp. 14-15, 164-5). During the show trial of Kamenev and Zinoviev in August 1936, Stalin himself helped to prepare the evidence (p. 338).
and it illustrates the weird picky pettiness of the man, something that helps us to form a more realistic image "who was this man Stalin"
He was always trying to correct people; he even tried to get Kaganovich to improve his punctuation, leaving Kaganovich to make the pitiful comment at the end of one of his letters: Ί will try to have periods and commas in future letters' (p. 52). Clearly, Stalin's politics were in many ways rooted in his character.
For national security/spy material, the Mitrokhin Archive described by the FBI as "‘the most complete and extensive intelligence ever received from any source" became available after the fall of the Soviet Union. This wasn't actually ever declassified -- the Russians never would have. Instead, KGB archivist Vasili Mitrokhin defected to Britain, bringing with him an astonishing trove of notes, covering Soviet activities around the world, revealing names of spies, collaborators, a torrent of information.
The archive runs from 1917 through the Gorbachev era, and so for example includes details about Lenin and his time that we didn't have, for example the arrest of the Russian Orthodox Patriarch Tikhon, and expropriation of Church property and suppression of religious dissidence. We've got early materials on Stalin's drive to "liquidate" emigre opponents for example. We previously knew this was happening-- viz Leon Trotsky-- but this is the material that explains who and how.
I'm late to this party, so I'll mostly rework this earlier answer of mine on Soviet historiography in general.
Much of the initial study of the Soviet Union in, say, the 1930s-1940s by Western scholars was undertaken by British academics with a broadly-speaking Marxist outlook (for example Eric Hobsbawm) or with a generally favorable view to the Soviet project (E.H. Carr would be a prime example here).
US study of the Soviet Union didn't really take off until the Cold War era, and at that point it quickly developed into what has been called the "totalitarian" school - namely that the Soviet Union was a "totalitarian" society, with everything controlled tightly by an all-encompassing central, ideological authority, and that in many ways this Soviet totalitarian system shared broad similarities to Nazism and fascism, and all were antithetical to Western liberal democracy. This outlook received funding and support from various parts of the US government, tended to use (but incredibly skeptically) officially published Soviet documents, but also rely heavily on defectors' accounts. It also had a heavy emphasis on political science, and treated Soviet studies as something outside area studies and academic history. Zbigniew Brzezinski would be a major figure in this school, and the school received broader popular attention through the writings of Richard Pipes (although he had his own Sonderweg version of Russian history) and Robert Conquest, the latter I should mention actually wasn't an academic historian. u/kaisermatias has a good summary specifically around how Pipes and Conquest interpreted Soviet nationalities policy.
The "revisionist" school that got underway in the 1960s and 1970s was often in hot debate with the totalitarians. It attempted to develop something more along the lines of a social historical study of the Soviet Union. Revisionists themselves were divided on the applicability of moral judgement to their work, and saw themselves as removing "bias" and providing "objective" analysis. They were at the very least "anti-anti-communist", but totalitarians often saw this as being outright pro-communist. A big focus in the work done here was on popular support for Soviet policies (rather than simply a focus on governmental bureaucracy), as well as "bottom-up" influences on government and party policy. The big names here were Stephen Cohen, Moshe Lewin, Ronald Grigor Suny and J. Arch Getty.
Now, an important point should be made here that until the late 1980s, both totalitarians and revisionists were working with an extremely limited set of documentary records. As mentioned, these largely consisted of official Soviet publications, defectors'/refugees' accounts, the Smolensk Party Archives (a set of provincial Communist Party archives from the 1930s seized by the Germans in World War II and then seized by the Americans), and the Harvard Study, which was a large-scale sociological study of Soviet immigrants in the US conducted after the Second World War. And...that's pretty much it. That's what everyone had to work with.
Anyway, revisionism isn't the end of the story, because there began to be a new focus in what has been called (for lack of a better term) "post-revisionism". This was heavily and often directly influenced by the work of Michel Foucault, and the influence of cultural paradigms (the idea being that both "top-down" and "bottom-up" perspectives neglected the decentered nature of power and cultural interactions). Stephen Kotkin is a big name here, and Francine Hirsch is a more recent example. I should note that Sheila Fitzpatrick is often considered a revisionist, but her more recent work tends to draw heavily on post-revisionists, and there isn't the kind of Cold War-era rancor between revisionists and post-revisionists that there is between totalitarians and revisionists.
Around the time that post-revisionism took off, in the 1980s, and early 1990s, there were massive changes in the availability of sources. Glasnost opened up the USSR, and academic researchers, both Western and Soviet (or post-Soviet) could now look through archival materials and gather oral histories. Dmitri Volkogonov would be a good example of a late-Soviet era historian here. Ironically, the totalitarian historians claimed that the opening of Soviet archival materials vindicated their point of view (the central authorities in the USSR drove most of the repression and political programs), but it was the revisionist who were actually doing archival research in the former USSR and updating their academic work accordingly, notably Getty (with Oleg Naumov) and Lynne Viola.
What is interesting is that the opening of Soviet archives, while it has enriched the field of Soviet history immensely, as not really shifted people's views. The totalitarians, revisionists, and post-revisionists all pretty much stuck to their guns and considered themselves vindicated, which I guess tells you something about academia or human argumentation in general. In academic terms, the debates seem to be more narrow, less about numbers or actions and more about intentionality (see the debates between Michael Ellman and Stephen Wheatcroft), or about what archival evidence there is for more specific revisionists' theories (I will link to an example from Oleg Khlevniuk critical of Getty below).
However, it's worth noting that the revisionist focus on society and the post-revisionist focus on culture have in many ways undergone something of a synthesis, and furthermore are getting displaced in newer academic research by what can even more tenuously be described as post-post-revisionist work, which I see Terry Martin classified under. So academically, there is less politics and less rancor in the academic study of Soviet history, but also more of a shift to areas less-studied by earlier historians, such as the Brezhnev era and nationalities studies.
So the TL:DR is that for major Western Soviet historians who established themselves before 1991, most didn't radically change their minds with the opening of Soviet archives, although those archival records provided much extra detail. But the academic historians who have come on the scene since the end of the Cold War have really plumbed archival materials to put together new understandings and outlooks that really move beyond Cold-War influenced debates.
Sources
Sheila Fitzpatrick. "Revisionism in Soviet History." History and Theory Vol. 46, No. 4, Dec 2007, available here
Oleg Khlevniuk. "Top Down vs. Bottom-up: Regarding the Potential of Contemporary 'Revisionism'". Available here
There was a nice thread on this some years back. I discuss one thing in the thread there, which is the fact that the USSR had actually been totally successful in putting nukes in Cuba during the Cuban Missile Crisis — they had over 100 nuclear weapons there. This was not known to the US until the 1990s, and came as a shock; the US was far closer to nuclear war then than it had even imagined (and it had imagined it was pretty close).
On atomic history: the opening of the USSR's archives has dramatically improved our understanding of exactly what kinds of intelligence the Soviets had regarding nuclear weapons, and how they used the intelligence. The latter is the most important revelation: they didn't just take the information and apply it, the way people often assumed they had. Rather, they were very suspicious of it (because who better than they would know about double-agents and misinformation!) and treated it as unverified for the most part. They doctored it up to look like it was coming from Soviet labs, and then fed it to scientists in the USSR who didn't know it was from the USA/UK/etc., who would then use it in harmony with their own research and re-checking. This is one of the reasons that very few Soviet scientists thought that they had any "help" from espionage information and considered their program to be largely indigenous. It also means that any argument about the intelligence information "saving" the Soviets time becomes more complicated; most historians now think it saved them less than a year, if any time at all, because information was not really what set the timetable of the Soviet nuclear program (access to resources and building of plants was).
On espionage history: we have far better knowledge than we ever did of who was actually a Soviet spy. The US VENONA releases helped with this, but the opening of Soviet archives, and the publishing of Mitrokhin's notebooks, really helped with this. It's now trivially easy to look up anyone who was accused of being a spy and seeing what the Soviets have about them; you couldn't do that before, and it has radically changed how historians talk about Soviet espionage and McCarthyism (basically the narrative went from, "McCarthyism was all nonsense" to "McCarthyism was largely nonsense, but some of the people actually were spies").