Username change are disallowed in ACP. So in UCP on Profile -> Registration details, your username is not shown in a textbox. If you have got an username shorter than the min value specified in ACP (relevant if value was changed after you registred), there's an error message saying "The username you entered is too short." - But how to enter a longer one?
So, for fixing this, before output of this message, check, whether the user is allowed to change his name.

(Hope it's clear what I mean.. )