I don't recall any of the South American dictatorships of the 70s ending by war or external pressure.

The case which I'm most familiar with, Uruguay, had been in a dictatorship since 1973, and in 1980 the military junta in power tried to give some legitimacy to its government, proposing a new constitution which would effectively result in a very flawed democracy. That constitution was submitted to a popular vote, and rebuffed, even with all the weight of the state press behind it, and many politicians who were against it either in jail or in exile.

Afterwards, the military junta recognized they had effectively lost popular support, and negotiated a elections in 1984 with a full return to democracy in 1985.

Edit: The sources I'm familiar with are all in spanish. An exhaustive and balanced view is given in Lincoln Maiztegui's 'Orientales 4' ISBN: 978-9974-643-70-3, dealing exclusively with the period 1973-1985.

Edit2, corrected dates.

Though oppression, freedom, and moral sense are all words that are extremely tough to judge, perhaps something that fits the pattern of your question is the 1989 fall of the Soviet bloc.

The transitions from Communist to democratic governments that all happened extremely rapidly in 1989 across multiple Eastern bloc states was unexpectedly quick and (though experiences in different states varied) remarkably peaceful and bloodless.

Romania's overthrow was violent and bloody, but in Hungary, East Germany, Poland, and Czechoslovakia, the Communist governments were all replaced, stepped down, or introduced reforms that very quickly led to their end, mostly without large-scale violence.

You can read Timothy Garton Ash's The Magic Lantern for a more comprehensive account. Though a historian, he was actually present at many of the events he writes about.

You're never going to be able to prove a negative. What you can do is go through many instances of changes in the position of power of oppressed people and see what happened. Slavery might be a good thing to look at because it has gone from being widely practiced to being functionally eliminated. I'm going to re-frame that statement to make it easier to evaluate, then provide my brief evaluation of it. I hope that other people with more expertise on particular times and systems of oppression will chime in.

I believe the crux of that statement is "Active resistance is essential to oppression, oppressors do not voluntarily give up their power on moral grounds."

I would argue that history has certainly borne this out. In the United States, the South had to be defeated in the deadliest conflict the country had ever seen in order to be forced to give up slavery. In the United Kingdom slavery was abolished in 1833, on the back of major slave revolts among many of their colonies during the course of the early 19th century. In the United States the civil rights movement was successful due to its ability to be disruptive - it's not as though southern states that were the centre of civil rights abuses changed their tune, the Federal Government forced it on them after resistance by civil rights activists.

In instances where we have people being oppressed - what impact did moral arguments have? I would argue very little. Again, going back to slavery - there were many moral arguments against the taking and keeping of slaves, but they seemed to have little to no impact. It seems that slave holders consistently found moral grounds on which to defend their slave holding. In ancient Greece Aristotle defended the keeping and holding of slaves as moral based on their nature - if they were worthy of not being slaves, they wouldn't be slaves. In the early modern period the Catholic Church defended slavery by arguing that it was ultimately a Christianizing force - and thus saved souls. Whatever damage was done to the actual people would be inconsequential compared to their immortal salvation. In Southern slave plantations they argued that they were providing food and shelter to people who would not be able to properly take care of themselves or participate in a civil society. While in all of these cases there were outside, non slave holding groups that would make moral arguments against slavery, this seemed to have no or little impact on slavery itself. The moral arguments against slavery were only powerful insomuch as they could galvanize a group to force another group to stop holding slaves. Usually, at the same time, economic forces (such as the Industrial Revolution) made slave holding less lucrative.

An example I can speak to in more depth is the improvement of positions of peasants during the middle ages. In the late middle ages, the social order was facing significant upheaval on several fronts. One was religious - the protestant reformation had occurred, and especially in Germany more peasants had direct access to scripture and suddenly saw that the things they had been told - that the social structure with them at the bottom was ordained by god - didn't actually appear very much in scripture at all. It seemed that Jesus actually liked poor people quite a lot, and favored fairly equal social statuses. Another was economic - plagues had reduced the peasant population significantly, and their work was more valuable to their lords than it had been previously, as man lords found themselves unable to fully work their land. Peasants began resisting - sometimes by arms, in several large peasant revolts, sometimes by moving around to get better wages working other lords' lands. Lords tried to curtail this degree of freedom by passing laws structuring the amount it was legal to pay a peasant, or outlawing peasants leaving their tenancy. There were many moral arguments for the (and against) the current status of peasants in this era, but ultimately it was these two factors - economics and resistance - that seemed to be the most in play.

So, speaking in probably too expansive generalities - it seems that there are a few key elements to overcoming oppression: an outside force depressing the oppression (the North in the Civil War, the Industrial Revolution in England), and resistance by the oppressed people themselves. If oppressors ever do abandon oppression for moral reasons, it is rare.

Didnt Franco's dictatorship is Spain transition to a constitutional monarchy relatively peacefully?