Well, I don't know the exact physics behind it, though I have caught sight of some statements proclaiming that an organism is pretty much an exact copy of itself after a while, with all constituent matter replaced (and that poses some existential questions on its' own), I think that likening a strictly material process to a philosophical/spiritual one that involves the singular unit that measures being is sort of like comparing apples and oranges. On the other hand though, this is an RP, and we could simply handwave soul-grafting/absorption/etc. as it simply working as such and not actually replacing the soul in question. I would still very much prefer souls to be imperishable unless subjected to specific effects, though. Keep that dream-logic in mythology and all.