They would need to change the Formation Screen sprite and portrait as well, just for starters (and I have no idea how that palette stuff works for the monsters, or if it just doesn't, and they each have separate Formation Screen data spots!) - you'd be best off just replacing the Simon sprite, as he's one of the few characters who has a spot in the Formation Screen data, and then latching his old sprite onto the unused Celia or Lettie sprites.
Unless you use the PSP, then you have to get rid of Valmafra or Orran, since Simon's sprite is too large.
Unless you use the PSP, then you have to get rid of Valmafra or Orran, since Simon's sprite is too large.