Welcome to the forums, I'm sorry it's not under better circumstances.
Unfortunately, I'm afraid to confirm that you're correct and you are not able to change your username.
When you create your account we do clearly advise that this is the case:
"Usernames must be 5 to 16 characters in length, lowercase, with no spaces or special characters, and must start with two letters. This lets you log into our website and forms part of your Plusnet email address. Once you've chosen your username you can't change it."
Though if it is something that you really need to do we can create a new account and move your services across to the new account, but we'd only look to do this after the expiry of any existing contracted period.
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