I don't think it is ever illegal to change one's own writing/evaluation. There's nothing "formal" there. It's mostly the psychologist's point of view of your therapy/evaluation. If they say they will change the statement that you have two heads, to one I imagine they will but anyone else looking at you in the future is going to see you only have one head? That's all that counts. If you don't like your record overall, it's covered by HIPAA privacy and you just refuse to share it (or don't mention that it exists).