oh! This is going to be interesting..

What if, Regan soul got switched? I mean the mark also got switched from neck to butt which might indicate a soul switching thing...

No, the mark near the butt predates the other one. She states that it appears to be a few years old, about five perhaps. That suggests that she accidentally marked him when they first met, and that his unrelenting pursuit of her is partly due to the mark.

mostly possible