This has me thinking that perhaps they could give achievements more of a cosmetic achievement by unlocking new hairstyles/marks and such. The only issue would be with face models since it would be odd to change those but it would be an interesting concept if perhaps defeating certain strong enemies or unlocking certain achievements added on to character customization. In which one could remodel their character aka haircut and other things with certain NPCs for a gil price, so it could also provide a gil sink.
It looks like that's a possibility, though I do wonder if it's perhaps something we'll be able to hide at will, since while it is a cool idea, I wouldnt' want my character to be sporting that 24/7 lol. It would be nice if perhaps we have the option to hide/show it.
I believe it's already been noted that all chars will be able to change their design and race because some option might not be available from the original, so they will be allowing everyone a chance to remake their character models and such.