Official comment
The "user name" has nothing to do with the user profile on the computer; it's likely that you used a different email address to register your Pioneer DJ account. You can login at pioneerdj.com (or rekordbox.com) to verify which account is in use and from there you can also see which computers have active installations with that license key.
You wouldn't lose any information in your library if you were to deactivate your account, however if you're unable to activate a second installation, or need further assistance in transferring the license to a different address, please file an inquiry ticket here and our rekordbox support team will be able to assist you.