So an example I though up is:before the undead showed up there was a cave bear that would kill the sheep of local farmers. The farmers where never able to find his cave so he kept killing their livestock. After the undead showed up he was killed by them. Of course the farmers only know he stopped showing up, until about a week ago. When the players meet him he is nothing more than a undead. Give him 10 Nero resist and 5 radiant vulnerability? Maybe his bite does ongoing Nero damage aswell?
Sure! That's a good reflavor.
Another solid one is to just completely patch over the old monster. Like just take the base template for an ogre, and call it an "undead abomination" stitched together out of multiple corpses. The necrotic/radiant thing is probably good, although it may be worth saying that in your world recent undead don't have those protections/vulnerabilities (otherwise Radiant Mafia is going to start an entire tier early).
I also agree the plot needs some sentient villains, otherwise the PCs are just going to get very bored of hack-and-slash. Undead are good villains for a bit, but they should never be the final villain of the setpiece (unless it's a vampire, lich, or some other sentient undead) since they're just not a very meaty villain.