Create a new WBSave for the map itself.
Change for all civilizations: warmaps in AIWars.py, city name maps in CityNameManager.py, stability/settler maps in CvRhyes.cpp, the spawn locations, cores, normal and broader areas in Consts.py, all spawns (cities and units) in Barbs.py, some territory-related goals in Victory.py and some additional things that I can't think of at the moment.
So it's a lot of work. What do you plan to do?