Character Replacing

Ok, so, I need something explained to me, please... Would it be possible to change an event to where Celes takes the place of Terra at the beginning of the game, and Terra is the one chained up that Locke frees... without simply swapping the sprites? Because I still want Terra to be the one who goes berserk. I figure the easiest way to do that would be to change some code in the game that just swaps the two's placement for the duration of those events.