In his 2009 book, How The Beatles Destroyed Rock And Roll, Elijah Wald argues exactly what the title suggests: that the Beatles destroyed the genre of rock & roll.

...replacing it with the genre 'rock', a different kind of music. Rock & roll was dance party music for the 1950s - music meant to be danced to, which was too loud and boisterous for the parents, and which was mostly sung by lead singers (Elvis, Little Richard, Buddy Holly, Chuck Berry, etc) usually with relatively anonymous bands. In contrast, rock had at its base fairly similar musical elements to rock'n'roll - but it was more focused on guitar (Little Richard and Jerry Lee played the piano, there were as many sax solos as guitar solos in the 1950s, etc), more focused on bands (after the Beatles, there was the Rolling Stones, The Animals, The Byrds, The Doors etc) that were seen as a whole...and less focused on making good dance music, and more focused on songcraft, saying something about society, and maybe making art.

As to when the terminology changed...as far as I can see the first edition of Rolling Stone magazine in November 1967 - which did as much as anything else to push the idea of rock - uses 'rock and roll' and 'rock' interchangeably, talking about the 'rock and roll music' at the Monterey festival, and talking about Chuck Berry's 'rock guitar'. So it's before that.

It's also before the Beatles got internationally famous - in the pages of the Mersey Beat newspaper in Liverpool, focused on the local rock'n'roll scene that included the Beatles, 'rock' and 'rock and roll' were used interchangeably - they talk about, e.g., going to France and seeing the Johnny Halliday Rock Show, not needing to explain that the 'Rock' in the name is short for 'Rock & Roll'.

In fact, the New York Times' first mention of a 'rock singer' is in 1958, in reference to Tommy Sands making a debut as a movie star. Which suggests that 'rock' was casually used as a shortening of 'rock and roll' before, for example, The Day The Music Died (i.e., Buddy Holly and Ritchie Valens dying in a plane crash).

As for 'rock' coming to officially mean that newer, different genre, Richie Unterberger in the book Turn! Turn! Turn! The '60s Folk-Rock Revolution points to an article in Billboard magazine as the origin of the term. On June 12th, 1965, Billboard published the article 'Folkswinging Wave On - Courtesy of Rock Groups' by Elliott Tiegel, which Unterberger suggests is the first prominent place where the term 'folk-rock' was used.

'Folk-rock' as a genre - The Byrds, Bob Dylan, etc - was clearly closer to the 'rock' camp than the 'rock & roll' camp, being less focused on making dance music and more focused on 'saying something', with lyrics that might be political protests or social commentary, or artsy in some way. And it was defined at the start as 'rock' rather than 'rock & roll' - 'folk rock & roll' didn't have the same ring!

So while people would casually drop the '& roll' since the 1950s, the advent of 'folk-rock' as a genre big enough to be discussed by an industry magazine like Billboard as the next big thing might well be the point where the '& roll' was first deliberately (rather than casually) left absent, where it came to start to represent a different genre to what had come before. (In contrast, the largely British music popular in 1964 in the US - the Beatles, the Rolling Stones, the Kinks etc - tended to be callled "Merseybeat", "beat groups", "British Invasion", and things like that, but there wasn't a name which distinctively dropped the roll quite like 'folk-rock' did). And after 'folk-rock' established the '-rock' suffix, you get other descriptions start to commonly be used using the same format - 'hard-rock', blues-rock', 'psychedelic rock', 'country rock', etc.