Show the developers that you would like to reroll.

[Dreadnought]

That would actually be pretty interesting. Where we'd all "die" and have the choice to reincarnate back as what we were or something different (of course keeping our levels, achievements, and items/gear like Yoshi promised). That way it would actually make in-game sense. It would also take away that odd concept that changing gender is any different than changing race.

To me a death is the easiest and most obvious rout for Rerolling characters.

if you chose to stay with your current model then perhaps you dont die, your picked up and carried off to town by moogles or something to be rehabilitated.

Also consider this:
there is supposed to be disasters and such to explain the changes to the zones. Well any huge disaster requiring re-growth (trees/Gridania) and re-building is going to take a large amount of time.
during this time your character would theoretically age and be in capable of fighting anyway needing you to take control of a new character.
Thats how i see it being explained.