[Edi]

The province removed and province added commands in the map editor just don't work as they should. So careful with the white pixels. If you forget to add one, you either need some script to fix all neighbors above that one or you need to redo all map neighbors above it by hand. Removing provinces is very much the same thing.

So you can use Ballbarian's script if you need to remove stuff. I expect an additive script would be similar, but comments in the script would be helpful for us non-programmers to tell us what it does and where.