If your name was initially auto-Guardian'd by the system when it went live, then it was deemed inapropriate for use as a BungieName. If you subsequently used your one-time name change to choose a new name, and that name was manually changed afterward to another Guardian'd name, then you've used your one-time name change and cannot change it. You'll have to wait until Bungie decides to grant another, if they do. Bungie has not announced any future options for name changes at this time.