As others have said, they're purely cosmetic at this point.

As you rank up further in your GC you'll be able to buy the books that allow you to change their classes with GC seals at your Grand Company. You'll also be able to change their outfits once they get to a certain level. Can't remember if it's 50 or 60.

At which point there is only one real option...

You go full bunny!