A: You've changed. B: No, it's you who has/have changed.
I'm not sure about whether we use the 'singular' or 'plural' in this type of sentence.
More example sentences: "It's you who is/are different." "It's you who need/needs us."
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Formally, as others have said here and in previous answers to similar questions, "you" is the pronoun that determines the number of the following verb, hence:
"It's you who have changed."
In colloquial usage, "It's you who's changed" is very common, and most native speakers wouldn't see anything wrong with it.