The Adventures of Sebastian Cole (1999)

Hank marries the divorced Joan, who's the mother of teens Sebastian and Jessica. After living with his new family for years, Hank announces that he's getting a sex change operation. The family deserts Hank, with Jessica going her own way and Sebastian staying with his mom. But Joan becomes an alcoholic, so Sebastian moves back in with Hank — now Henrietta — who turns out to be far more supportive than anyone else.