Been awhile since I've posted someone on the site. Hopefully, you find this bit interesting.
From the manga series, "Toriko", the intrepid Gourmet Hunter heroes work on cultivating a special ingredient called "PAIR" that can become soup. Long story short, apparently drinking it causes the person to swap genders.
http://www.mangainn.me/manga/chapter...er-320/page_14
It seems like it happens from the neck down, but that could be just me.