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They're not 100% what you're asking for, but these are related:
Vampire Refugee / Resist the Beast: Bob is turning into a vampire, and tries to resist it.
And Then John Was a Zombie: Bob used to fight vampires, and now he is a vampire.
I Know Youre In There Some Where Fight: Alice (who typically knew Bob beforehand although an All-Loving Hero may also qualify) gives Bob a motivational speech to help him resist the vampirism.
Not Brainwashed if its a situation like Alice revealing to Bob that she agreed to be Reforged into a Minion
Two characters used to know each other well. They meet again after one of them has undergone some sort of transformation, like being turned into a vampire, and the other one is usually shocked by the change.