if you dont need the old door then you could always just snap it off to get better acces to clean it so you can put it on your new one, otherwise use a cutting knife and try to pry it of little by little, if that doesnt work then try soaking it in boiling water for a while and the see if it is loosened, make sure you dont put the whole door in if you need it though as it may deforma nd just put the metal bit in.