I grew up on the original Star Trek (I'm so old I even remember seeing it during the original airing a time or two), so there was a long time where all I had was the original series, then the animated one, and finally, the movies. During that time my friends and I, and those we met at conventions, were sure Vulcans didn't have emotions, or if they did, only vague ones, and the reason it was tough for Spock was because he was half human, so he had emotions full-blood Vulcans didn't have.
There's a lot of support for this, for example, these quotations:
From Where No Man Has Gone Before:
Kirk: Dr. Dehner feels he isn't that dangerous! What makes you right and a trained psychiatrist wrong?
Spock: Because she feels. I don't. All I know is logic. We'll be lucky to repair this ship and get away in time.
From Dagger of the Mind:
Mr. Spock: Interesting. You Earth people glorify organized violence for 40 centuries, but you imprison those who employ it privately.
Dr. McCoy: And, of course, your people found an answer?
Mr. Spock: We disposed of emotion, Doctor. Where there is no emotion, there is no motive for violence.
From Spectre of the Gun:
Spock: My feelings are not subject for discussion, Doctor.
McCoy: Because there are no feelings to discuss!
From The Making of Star Trek by Stephen E. Whitfield, Page 226:
Vulcans long ago concluded that emotion was dangerous, set about to repress it and replace it with logic. Century after century, through practice and custom, they repressed emotion until they became almost incapable of it. Logic became breath, sensation, as uplifting and delightful as the emotion it replaced....
Because of his Mother's origin, however, Spock does have a human side to his personality. A human side with emotions.
When Star Trek: The Motion Picture came out, what seemed to be the common understanding at the time was that the kolinahr was a ritual where, as Memory Alpha states, "all remaining vestigial emotions were demonstrated as purged." Again, this indicates that Vulcans had few, if any emotions. Spock was going through it because he was purging the emotions from his human side.
But later in Star Trek, the story is different. In Sarek Troi says, "Vulcans have the same basic emotions we do. They've just learned to represse them." Later Sarek states, "Vulcan emotions are extremely intense. We have learned to suppress them." (The episode is about Sarek dealing with Bendii Syndrome, which is "a degenerative neurological illness affecting a minority of elderly Vulcans. Initially it is characterized by wasting, weakness, fatigue, fever, and a gradual but accelerating loss of emotional control, with victims exhibiting sudden bursts of emotion.")
There's a shift here, from little to no emotions in the original to intense emotions that they repress. It's a relatively easy retcon, since most statements, up until then, were more in the line of casual conversation.
When did this change happen? Is there any thing to show it was a gradual change, or was it a change that happened within one or two episodes?
Note: For once I'm not asking about an in-universe change, since this was clearly changed in terms of writing and was retconned (not a hard retcon, honestly) so the idea was no longer that they didn't have emotions than that they had intense ones and repressed them. I know about Surak, but, again look at the quotations from the original series. At that time, in the 1960s, they only had, if anything, vestigial emotions. But in ST:TNG, in the 1980s and 1990s, it's different. So when did the shift or retcon happen? I think it was in the episode Sarek, but I'm not sure and I'm wondering if I missed something along the way.