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Okay, then your forum name would be changed, but your FFR account would be totally separate, causing the system to break. It's easy to change your name on the forums, but FFR also integrates the game usernames with the forum usernames. There would be a bunch of other databases that would have to be changed, not to mention quite a few tables in each - which would all have to be done manually.
Sure, the manual work isn't too bad when it's done rarely while under special circumstances, but doing that for anyone who wants it would be very time consuming and it's just not worth the time of the administrator who would have to be constantly updating tables which are easy to break as it is and could cause lots of other problems.