All questions regarding mapmaking for Heroes of Might and Magic 3: The Shadow of Death.

Could you tell me if it's possible to copy an entire level into another map?
For example, to insert a Dungeon from one map into another map where it didn't exist before.
I think there's something like that in the WoG editor, but I'm not sure. Will changing the map in the WoG editor affect it?

Answering your question and my previous one, I'll say - it is possible to copy a map!! It's strange that people on this specialized forum couldn't help us with this most important question for mapmakers.
It turns out it's quite simple. This function appeared in the WoG map editor version 3.3, and it allows you to copy the entire landscape, objects, etc., both within one map and to others, on an unlimited scale, whether on the surface or underground! In your place, I would install WoG, copy, edit, and save the map (in the specifications, set the checkbox to SoD). Then, reinstall SoD on your PC, and that's it. I haven't tested this method, but it seems like it might work.