As ruby said.
Though if you must/want to keep your characters you have on your accounts along with the posts on the forum and the settings. We should be able to set a new username for you, however though, it must be a unique username that no one else use at this moment.
If you got a unique name though it can be changed and a mail will get sent to the mail you registered with. In the mail you will have the new username and a temporary password that you are able to change once logged in again.