Dorian Requiem
I think shes talking about having the spirit of someone who is incarnated leave the body its in so that the unincarnated spirit can take that persons body.
The flaws I see in this is the probability of this being done is so slim I can't imagine a working diagram of it to properly show how small the odds are. Secondly, I think that the more developed neural pathways of the persons body will likely have some impact on the personality of a new/foreign spirit inhabiting the body.
Kezaih, I think Knola called them "Walk Ins".
Yeah, that would be a walk in.
I do think it's happened before, but it involves no 'killing'. And from the cases I agree might have happened, it's usually a deal forged between the inhabiting spirit (usually very depressed or not wanting to live) and the spirit to move in (wants a physical body badly).
Odds of it happening aren't nil, but they are slim. Odds of someone getting their spirit kicked out of their body and taken over by another spirit like in the movies? Extremely slim, but possible.