Neville, check out sefer Dor tahapuchos, which is all about transgender issues. For social things, like where to sit in shul, a person who changed genders, whether for pikuach nefesh or stam b’issur, sits with the one he “changed” into, but for all halacha issues, like kidushin, etc, he remains a man according to everyone. People like to throw around a tzitz eliezer who never would have dreamed of people changing their gender the way they do today; he writes about babies being born with defects and having doctors do surgery to determine the gender, etc
