Well, doesn't really matter. Either the threat side of the NPC hasn't really surfaced yet, in which case who cares? Or the player is interested in the NPC precisely because it is such a cool threat, in which case you can bet that the new PC will be a better threat for the other ones than your NPC ever was.
If there is some specific, weird, unknown nature that the NPC had before PC-ification, feel free to give the new PC a special move. Doesn't need to be anything too fancy.
Cannibal - when you see blood or exposed human meat, you're under fire from your hunger - lose it and you can't do anything but pursue this tasty morsel in your sights.
As a general rule, say yes as often as you can, and think of how to resolve it afterwards.