In most isekais I've read, the Og character dies, allowing the Mc’s soul to take over.
But even if that's not mentioned, I'd imagine they either die or switch souls. Cus a soul can't just dissapere to nowhere.
Yup, most just disappear - but this one doesn't^^
https://www.mangago.me/read-manga/the_one_within_the_villainess/
Sometimes they do swap - like in "The Duchess' 50 Tea Recipes" or "The Big Shot's Darling Returns" - but that swap mostly doesn't play a big role in the stories. I've also read one where OG villainess is around in kinda ghost form (only to be seen by fl), watching fl, repenting and leaving on her own...
There are several possibilities. The most common is that og villianess died, but I have also seen some in which both souls are in one body or the souls of the villainess and mc swapped bodies.
When the MC replaces the OG Villianess, where does rhe OG Villianess soul go? Is she dead, does she just dissappear.
It would interesting to read a reverse isekai, where the OG villianess comes to the real world.
or the OG Villianess comes to the real-world for a bit and then returns to world and changes the story