In the spirit of ID and the hypothetical posting:
>weeb scientists invent magitech that can TSF people to their specifications
>they are using it to Make Waifus Real
>snatched off the side of the road one day, dumped back in the body of <insert waifu> days later
>you're not the first, authorities explain what you're in for
>your mind is currently your own, but is inclined to 'slip' towards thinking like the character
>you can shake the feeling if you concentrate, but they tend to creep up on you; you might not notice you've been thinking more and more like her for hours because it happens so gradually
>you need to stay aware of this, because if you act on these slips by actually DOING stuff in character, they become more frequent and harder to shake, and your 'baseline' shifts closer and closer to her
>bigger personality shifts are easier to spot and resist, but if a character is quite similar to you anyway the process is much more difficult to stall
>worse, giving in feels good. It's easy to justify small changes that add up in time. Fighting the urge to dress in full cosplay 24/7 is difficult, but maybe you start dressing like a girl to dull that itch, then before long, like a "casual alt art" design of the character without even realising; that's just the sort of outfits you feel comfy in now...
>your memories are safe, but keep slipping and eventually you'll be indistinguishable in personality and behaviour to her as well as in body

When you look in the mirror and see who they've turned you into, who are you hoping to see? How fucked are you if it's the last character you got off to or the main girl of the last media you consumed?

(I have three drafts of a story based on this in various states, a 2nd person one in which (You) become Megumin, a 3rd person take in which a protag who doesn't know her is scrambling to research her so he can actually pick up on the changes, and a one where the protag does knows but very much does not want to be Ui from BA).