Multimap module need to fix a map that is in a scenario

Is there a way to alter a map and have it show up in a saved scenario? Or do you have to rebuild the scenario from scratch?

It depends. Are you simply swapping out the image for a new image? If so, that works fine.

If you have map-related module configuration items like Zones, Regions, grids, etc…my memory is hazy on if those can be altered and behave correctly when using older saved games that pre-date the changes.

Yes just swapping out the image, I assume it has to keep the same name in order to work or can it change also?

Precisely the same name–including case-sensitivity of the filename and suffix.