Interesting! I didn't know about that. When I programmed the campaign editor, I ensured that different types of farmland do not appear in the same province, e.g. if you add legendary farmland to a province, the original improved farmland will be deleted automatically. I have always assumed that having two types of farmland in a province will crash the game.

For the moment I guess you will have to do this manually if you really want to do it.