As mentioned above you can't really change your actual log in username. The username you type in for the game and to log into here will never change. You may however change your username here on the forums only.
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To change your forum name from your defult username, you go onto your main profile. From there you go to your left hand side tool bar and click on "Forum Profile Information"
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From there the section that is right under your Log in username is what you use to change your profile name. Just note that when you are making a username, make a username that isn't so common or familiar names such as John or Valerie, etc. when you often have accounts like that, messages might get transferred wrong to another user with the same name.
But if your main intention is to change your log in then I'm afraid there really isn't a way. As everyone else I think your best bet is to just wait till registration opens up again.