Also known as: エックスチェンジ
X-Change is a erotic game series by Peach Princess, where you play a young man named Takuya, who, due to drinking a strange science experiment, ends up being the unwitting guinea pig in a gender swap experiment, which is the source of the title of the series.
The game was followed up by a sequel, X-Change 2, where Takuya ends up accidentally turned into a girl by a machine, as well as a third, and final, part, X-Change 3, which completed the storyline.
The game's premise in all three games, is deceptively simple, that Takuya has to find a cure for his transformation.
This is made more difficult by the fact that a lot of characters in the game want Takuya to stay a girl, and the endings reflect that, with only one or two endings leading to the cure.
The series was followed up by Ying-Yang! X-Change Alternative, a new slant on the series, which uses a different main character and setting, although keeping the concept of the sex-change potion.