The main theme here is the triumph of evil over good, with my good guy character getting turned into a bad girl. (Just a little disclaimer, I use the terms girls and boys for flavoring, not implying any age-inappropriate activities are going on here. There doesn't have to be sexual intercourse involved, depending on ages of the characters)
So, here are a couple of options:
- Modern realism: Naughty evil stepmother doesn't want the son to inherit the family fortune. As a male, he is meek, nerdy, gets picked on a lot by the jocks, while girls laugh at him. She exploits his infatuation with her, and the fact that his life sucks, to turn him from loser boy to hot girl, with the mantra that "pretty girls get anything they want". She also "trains" the new hot girl to use her sensuality to manipulate boys. Character decides life is better as a hot girl than as a loser boy.
- Modern realism: Nerdy boy happens to be son of the police chief. Hot girl he's sitting next to in class is involved in criminal activities. Like the above scenario, he is meek, nerdy, gets picked on a lot by the jocks, while girls laugh at him. She exploits his infatuation with her, and the fact that his life sucks, to turn him from loser boy to hot girl, with the mantra that "pretty girls get anything they want". She also "trains" the new hot girl to use her sensuality to manipulate boys. Character decides life is better as a hot girl then as a loser boy. "Girls" become partners in crime, using their sensuality to get away with being bad.
- Modern fantasy Young male witch hunter bites off more than he can chew when he goes after the Queen Witch. She captures him, seduces him, then turns him into a beautiful young woman. Naturally, she has to train her new pet how to be a hot girl who uses her sensuality to take advantage of males. Not being an actual witch, and being familiar with how hunters operate, the new girl can help her mistress/lover track down and eliminate other hunters.
(The alternate plan is the evil wizard is male, who can appreciate how much more useful his pet would be as a beautiful young woman, with the potential for some really humiliating stuff for the straight male hunter.)