There is no way to modify ones username via the normal control panel. The only way to do this is to create a new account in which case you'll lose your post count and join date will be the date that the new account is created instead of Oct 2010.
Also if you do this there will be no connection between your "old" posts and your "new" posts as these will be under different user names.
Probably the best thing you can do for now is include an avatar of your latest wheels and update your signature (which I know you've already done).
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