I think you can say either "who" or "what", depending on the context and what you want to say. Anyway, I think it's usually "what" in those kinds of sentences.
I'm not Homer Simpson anymore. I'm not who I used to be. I am Mr. Thompson now.
Hello. Thank you for your comments. I came to a conclusion that "what" is correct. Some people use "who" and they may insist that "who" should be right. I agree to it. But I will use "what" from now on. Thank you all!