I think it might just be that he's been a vampire so long that if he were changed back to a... whatever he is... it would effectively make him a new person even if he retained his memories. Sort of like how becoming a vampire made him a different person in the first place.
Also, I just realized something. If Malack is 200 years old, that means that he could easily have been involved in the adventures of the Order of the Scribble, and could be an invaluable sorce of information.