Yeah, this probably wouldn't be possible. I believe you could make a new map in map maker, name it the same thing, and replace the original map, but this would cause a lot of problems since things wouldn't be in the same spot and such.
If someone did manage to get into the original map files, it would work more as a downloadable mod than an update. I wasn't here when the game was still being updated, but I believe there would be popups saying to go get FH updated and it isn't possible to do without the source code.
I like the idea and it would be nice to have a revamp of the maps, but sadly it isn't possible. I've made a whole map pack that has remade the original maps, but it only works as a downloadable map and won't replace anything.