KJM:
And vice versa as well, why not? I could imagine a story when a male turned female spanks a female turned male, something like that: F(M)/M(F). If you read the rejuve series started by Lurking Dragon, the criminals were rejuved not only to their own gender.
Well, in the "Melody" series, certain male criminals who had committed violent crimes, especially sexual ones, were rejuved as females via intentional "y repression," yet I'd venture that most Penitatas (criminal) rejuves retained their own genders, as did the other types. (However, "y repression" sometimes occurred accidentally during rejuvenation, while some limited emergency rejuve chambers resulted in all males treated within them ending up as girls.)
Plus I don't believe that females were ever transformed into boys via rejuvenation; the gender alteration went only male-to-female.
Still, the changing of genders is hardly unknown as a science-fiction or fantasy theme, it could certainly be employed within this current Challenge, I'd venture...

--C.K.