If I build a base on the Plateau, then move to the Foothills, will my base still be there if I move back to the Plateau? Or will I have to start from scratch?
Your base will be destroyed. All of your outposts will be lost. Your enclave relationships will be reset. All looted buildings will be refilled and unexplored.
When you leave a map, all data saved with that map will be lost. You will be compensated with influence and materials for lost relationships and buildings, respectively.
Going to a new map is a completely fresh start, with the exception of characters, inventory, resources, and influence.