Let's say that I happen to have a dead body in front of me (don't ask where it came from or how it gained the "dead" status). At the time, it makes sense to make it into a zombie using animate dead due to the abundance of flesh still clinging to its bones, otherwise ruining a perfectly good burning skeleton.
At some later point in time, I learn the spell decompose corpse. Would this allow me to transform it into a skeleton? Basically, are there any ways of altering an existing undead into another type of undead, hopefully using low-level spells like the above or restore corpse?
I have asked other questions about restoring the materials of a destroyed undead for reuse, or causing an undead to become dead without destroying it, or if undead still count as corpses, and all were answered "no".