Probably one of the most convoluted gender flip mechanics I'd ever seen. I mean Ranma only needs a source of non-hot water which makes up the vast majority of the Earth's surface.
... also I don't see that person who would always comment that Eito should become a girl. So I'm gonna go ahead and do it first -- after seeing how adorable and cute Kei is regardless of his sex? Like almost nothing of substance changing really, if you think about it? I'd find it more interesting if the decidedly male moody mcbestfriend got it instead.