I think it makes sense.
Before this when granny died and whished to be reborn and be of help to Fushi the black hooded man said that she'll need to wait before the gates of heaven and he'll come for her before the guards take her away, or something along those lines.
Which would mean that the souls of the living linger around after they died until they're taken away.
And from that, I'd assume when Fushi made the copy of the body, the empty husk, the girl's soul could take over the body and so she essentially resurrected, with the same soul, but with a healthy body.
Also I'm guessing that the people he can turn into, instead of being taken away, they reside within him, I think that's why the prince can see the bear's shadow, so he'll probably be able to bring his companions back to life, but lose the ability to turn into them while they're alive (That's probably why black hooded guy keeps this as a secret).
This'd also explain why he can't take the form of living things until they die, and how the knockers take away his transformations.
Also because of the prince I'm guessing there are some kind of wandering souls/spirits too, and those are the people he's talking to, like the guy whit a spear in his head from a few chapters ago.