Pretty much like FF6, except you need to change screens for your leader to be updated. Once it's set up for one character, it's really easy to get the others in it too.
It would take a while to get it all done, but luckily it's mostly copy and paste.
Note that your leader can only be changed by PHS, NOT order in the menu. To get Aeris and Cloud swapped, I had to unlock Cloud, replace him with Aeris, then exit the menu, go back in, replace Barret for Cloud.