It is not possible to make username changes, it is not only in the forum the username is actually used, there's parts of the backend that tie forum and hosting together that would be affected for instance. If you have a previous one you can login to the portal and from there change which forum account is linked in with the hosting however.
Once logged into portal go to "Get Support" -> "Visit Forums". This will provide you with information about currently linked forum account as well as give you an option to switch to another existing forum account.