Personally I would "fix" that by adding two characters, one (the undead one) would just be deactivated and therefore invisible. When he reveals he is an undead I would pause, deactivate the one that turns into and undead and activate the undead one, and start the time again.
Maybe some of the mods, like the jravens toolkits have this added, but as I dont use any yet, I don't know. So that is how I figure I would do it.