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Not a stock phrase, but We Want Our Jerk Back! and "Flowers for Algernon" Syndrome seem to fit the situation.
I think what I'm searching for would be a Super-Trope of those more specific tropes. It would also cover situations where, e.g., Alice (not me!) laments that her best friend Carol, as a result of dating Bob, has become more and more like Bob and less and less like Carol. Or where Ted goes from being a jock to being an RPG geek and his jock friends feel they're "losing" him. In general, any situation in which a character, rather than accepting that someone close to them has changed in personality and/or interests, or trying to meet them halfway, complains that the other is no longer the person they knew (and, they believe, should only be).
Yeah, I see the difference there (plus the other two usually come with a Snap Back, while what you're describing doesn't have to).
Do we have a trope for when a character claims that their friend, relative or lover has changed, personality-wise, beyond recognition? The typical Stock Phrases (not listed in this wiki) are "I don't know who you are/who (s)he is anymore," or "What happened to the X I used to know?"