If you do that.. Would we have to edit every field file too in the flevel.lgp?
It would have to be done for every level. Playing as whoever is in the top slot has a few problems though, like in Mideel when you find Cloud, the game crashes unless you play as Tifa.
I have found a code that lets you switch characters after talking to them, for example with the Aeris patch, i gave her text on the highwind, and if you pick yes, you play as Aeris. Its quite fun actually lol.
EDIT: Sorry, almost ignored you
No, it wont work on the world map. You have to do it for EVERY level. Although its just a simple copy and paste, and in some cases you may have to do a little tweaking.