Spells R Us – Dream Girl 2
This may have been a bad idea. On changing gender, most people end up taking on that gender's societally expected mannerisms, usually because The Mind Is a Plaything of the Body. Of course, things get worse when Terry takes a Love Potion in front of Crissy. The Old Man gives them the spell and "forgets" to tell the boys that for one night, they and the other frathouse students will become girls in body and mind. Then later, Dennis is under the spell and "Denise" falls for Terry and thinks Crissy is trying to steal her away. A small collection can be found at this angelfire page and this archive of TF stories. As the story progresses, the customer will have either a Gender Bender , Animorphism , age progression , age regression , or some other sort of transformation.