Just bare in mind that, the only undead that were made "to be undead" are the special ones you can dig up.
If you didn't dig it up, no one said "this is how I want the undead creature to be"
The DM was designing a monster to be fought by PC's,while it is alive.
So, any requests to change none dug up undead is just a no, because it's a request to change the monster, not the undead version.