Was looking for something simple and remembered this classic Doujinshi (Oppai Soap de TS Ochi! by Omuai Okashi) 
One location simple progression, should be a good pick.
Basically Guy gets into soap land where one of the girls applies transformative oil onto him, but not with hands instead she uses her body, oily and soapy body.
Eventually boy becomes a girl and corrupted enough to work at the soap land.