You realize that upthread, you gave no indication that you gave a whit what the player thought of his character's change of behavior? You just said that if he described one thing, and then did another, he wasn't role-playing.
I'm not sure where the confusion or contradiction is. If he describes it to the GM or keeps it in his head, a person who Role Plays has an idea of how that person is, and they consistently portray that person.
To an observer, it should be obvious that this is the same person from scene to scene.
I've seen more than one paladin have the fantasy equivalent of a psychotic break. Being a paladin is pretty stressful stuff.
If the player can justify it, whatever they said before, it's role-playing. And, to be honest, I'm okay with pretty thin justifications.
I can't stop you from accepting poor acting as roleplay. Here's the litmus test: If you were reading the adventure as a book or TV show, would you really accept crappy acting as good writing?
Characters evolving is generally accepted. Characters acting out of character for no good reason is consider crap writing in most forms of fiction.
That isn't who the character was, yesterday. I don't have ownership of another player's character. I don't get to say who that character is.
According to the old alignment rules, as GM, you do. Ye olde timey XP penalty for changing alignments is such a rule. Acting out of character is a judgeable and penalizable thing.
There should always be room for player/GM discussion about "I don't like the way I defined my character and want to change it." That's no different than "this feat I took was totally stupid, I'd like to change it."
But that's different than crappy acting.
To me, the very point of RolePlaying is to say "I'm playing Dudley DoRight as a Paladin" and to consistently play that way or evolve him, such that your PC is voluntarily constrained from certain options like being evil, backstabbing your friends, consorting with demons. To wake up the next day and do those evil things because it is convenient and a "smart move" is out of character and is not RolePlaying.
It's no different than your girlfriend putting on the naughty nurse outfit and then spending all night talking about work (once again, assuming she is not a porn star) while she engages in boring chores and nothing naughty occurs during the roleplaying session.
It should also be noted, that despite my strong opinion, unlike social skills, there is no game mechanic to enforce it. At best, there WAS the alignment change penalty (which its design intent is most likely as I described it upthread).
So, I cannot mechanically enforce roleplaying nor measure how well you are doing it. Or even fairly discern if your PC's behavior change is "in character" compared to Bob's thinly veiled excuse to turn evil so he can jack with Jane for getting his horse killed last week.