Current questions and answers regarding the map editor.

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

It's impossible to replace a creature without any impact. However, you can replace a Yeti or a Black Knight, which are rarely used on other maps, so the impact will be almost negligible. You can create a copy of the town and assign a specific ID to the desired building, so it's unlikely to have any effect.