Scientology has their own beliefs on this that's not quite the same but close enough to what we are talking about here. Instead of soul replacement or what I mentioned as something similar to demonic and spirit possession which would be a mind/body take-over like E. Montgomery's "walk-ins".. Hubbard believed that the spirits of aliens attach themselves to humans and that only Scientology can detach them from us.
Xenu (/ˈziːnuː/),[1][2][3] also called Xemu, was, according to Scientology founder L. Ron Hubbard, the dictator of the "Galactic Confederacy" who 75 million years ago brought billions[4][5] of his people to Earth (then known as "Teegeeack") in a DC-8-like spacecraft, stacked them around volcanoes, and killed them with hydrogen bombs. Official Scientology scriptures hold that the thetans (immortal spirits) of these aliens adhere to humans, causing spiritual harm.
https://en.wikipedia.org/wiki/Xenu#cite_note-savino-7
From my understanding, Scientologists do not believe in things like demon and spirit possession but they DO believe in spirit attachments... specificallly the Thetan alien attachment where it literally adheres to the human spine/back. I've seen spirit attachments on people with my own two eyes clinging to the back of the human host. I've always known them to be very low level monstrous earthbound human discarnates... not aliens.
The lower the level of the earthbound spirit, the more monsterous and animalistic they look. I'm guessing that Hubbard saw them too but he had a different interpretation of who an what they were. The Gnostic pov on this would be that they are archons.
Yes, this is pretty crazy stuff and it's definitely in the You-have-to-see-it-yourself-to believe-it dept.