I could go for a race change option, though I don't see the necessity of it being a paid for privilege. I suggest that a race-change could be the result of an epic quest that requires an amount of effort akin to a Magian Weapon Trial to complete. That way it can be worked into a plausible storyline (something... Warriors of the Crystal... something... racial aspect... something...) and be a significant undertaking that people wouldn't just be able to do on a whim (which ppl with money to burn can do with a paid for service).
I don't see how RSE can cause a problem? Surely it's just "Select Race Change" -> "Dequip character" -> "Change Race" and then you don't meet the criteria for equipping it? Any KIs that are race-specific are just lost (eg Promyvion lights) and need to be reacquired.
Also, the conquest caravan NPCs should incorporate a hairdresser that can change your hairstyle and colour for a nominal amount of gil!
Personally though, I wouldn't mind starting a whole new character of a different race from scratch on the condition that my Rare/Ex gear can be transferred between all characters on my account.