Transformation of undead in the Temple of the Underworld.

I wonder, in what way exactly? I simply don't see any other options.

Here's an option: make the transformation button inactive in the interface. Instead, after building the Temple, add a scripted transformation spell to the hero's spellbook (or allow activating a spell already written in the book) that transforms the town into a Necropolis. I've seen such a spell in the NHF 0.84 mod and in Reanimation Mod 1.2. The town transformation would have conditions, such as requiring money, resources, or the hero's level being higher than or equal to the town's level, etc.