I don't think you can unless you email the ed and ask him and give a valid reason why. I'm sure I read that somewhere on here. Hopefully a more experienced person will be able to
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All you can do is to re-register (using a different email address). Your old posts will remain under your old username unless you can persuade the Ed to close that account, or unless you can get a moderator to ban you under your old name. (I've 'banned' a couple of members at their own requests).
I asked back in May if I could change by name from upper case to lower case as I look as if I am shouting at people. Did not realise that upper case was classed as shouting when I registered and most people use lower case. Quite happy to keep the same name. I received an email on 9th May saying they would contact me shortly. Never heard anything after that.
in the end I just changed the gravatar to an appropriate form in keeping with merlin the magician, I ended up with merlion beause of a typing slip, and we are where we are suppose-I'll leave it at that!