変身 (
henshin) means things to do with transformation, but I can't help but think of Power Rangers when I hear that term. ("It's morphin' time!") I'm sure I've seen it in plenty of transformation stories but for some reason it's not standing out to me at the moment.
You have to be careful, though, because
henshin could just as appropriately be applied to changes in age (like a loli becoming a young woman or vice versa because of a spell), to changes in build (like a flat-chested girl becoming an incredibly busty girl instantaneously because of a magic potion she drank), etc. It's a general term with a wide range of possible hits if you try to search by it.
What I usually see in stories where the girl is turned into a succubus or a wolfgirl or some other kind of "evil, naughty girl" against her will is 生まれ変わる (
umarekawaru, "to be reborn"). It's a compound-verb of
kawaru and
umareru;
kawaru is the intransitive form of "to change" (i.e. it's what you'd say for the English sentence "people change" and not the sentence "people change the world"); and
umareru is the passive form of "to give birth to", i.e. "to be given birth to," a.k.a. in common English "to be born". So basically "born-change", hence "reborn." Hope this makes sense.
As for what
genre these stories are referred to (like how we have the terms
futanari,
bakunyuu,
rori, etc that you can reliably search by), I honestly couldn't tell you off the top of my head despite loving these sorts of stories myself. I usually wind up finding it quite by accident in the commercial magazines that have more paranormal leanings (like Comic Unreal and KTC's Slave Heroines series); otherwise, I find it incidentally while searching for other genres that have some overlap, like psychological corruption (e.g. the girl becomes a sex-addicted slave, 肉奴隷
[nikudorei] or simply 奴隷
[dorei]). If I think of anything later I'll be sure to edit this post and let you know but for now I'm drawing a mind-blank for any "index term" for physical corruption stories.
The final thing I'll say is, even if you
do find an official name for the genre, that might not help you out very much. When you do a google search in Japanese for some of the better-known but still niche fandoms, you can expect to get fewer than 5,000 hits in most cases and a lot of the times the top 10 pages are nothing but links to 2chan and random fans' weblogs.
Only bumping this b/c of
this newer thread where I saw your reply with a link to this older thread, and then how I saw this discussion of what terms to try and search for.
