Oh, wait, so, do you mean writing fic about, say, Frodo, but make it so Frodo's always been a lady hobbit mentally and physically? Then I guess you could do just about anything with it - whatever feels right or you feel like, from making her a crossdresser (like a variation of the "Frodo was secretly a girl all along" scenario) to writing, well, just Frodo in a dress, basically, or even subtly changing his personality based on the different imprints she would have received from hobbit society, her awareness of being a contrast to the male hobbits.

Or did you mean "ZING! You now have the mentality of a girl!" type of genderswap? That'd be interesting - and challenging - to write! Rumiko Takahashi did it once, and it turned out Ataru with a female mind is a gentle, wilting sort of horny lesbian.