Posts from Current questions and answers regarding the map editor.

Will a map created in version 1.6 work in version 3.1?

Please tell me, is there an editor for macOS, and if so, where can I find it?

Is there a simpler way to do this?

How can I replace a creature in a town so that the change only affects that specific map and only in that particular town?

Yes, it should.

No, definitely not. There won't be an official one, and fans are unlikely to take on such a task. Possible solutions:
1) install a Windows emulator
2) if you don't need to edit the landscape, you can use an archiver that supports zip archives, i.e., unpack the map and manually edit everything in a text editor.

You can't replace it with something that has no influence at all. But you can replace the Yeti or the Black Knight, which are rarely used on other maps, so the influence will be almost negligible. You can create a city as a copy and assign a specific ID to the desired building, so it is unlikely to have any influence.

Why change the Yeti? You can simply replace the creature in the building properties with the desired one, but first, copy the building to your map.

Which creature are you planning to replace?

For example, I decided to place an Academy town on the map, where archers, crossbowmen, and marksmen will be recruited instead of gargoyles.

There's a mod that adds the Naga town from Heroes 6 to Heroes 5, based on the Elves, but it's not fully finished. I want to replace the Druids with Water Elementals and maybe the Green Dragons with Water Titans (from the NCF project).

I suggested the most optimal method: cloning the town and modifying the clone to have the ID of a Yeti or Black Knight (if you need 3, then either a Manticore or a Death Knight). These creatures can be replaced with anything (even from NCF), and we won't have any penalties, with minimal impact on other maps.

Then there will be problems with checksums. Otherwise, there will be no impact. You can create a town consisting entirely of creatures from different factions.
But, as they say, it's up to each player.