Regarding whether a username can be changed or not, would depend on whether or not you have already updated your username for your account, or if you have already registered your Genesis League Goals account by using an active Hive account.
Users who initially register an account on the Genesis League Goals website (excluding with a Hive account), called a lite account, will start with a default name for a lite account, such as a few of the examples listed below:
The names and numbers all vary for each user and provide a unique username until a user decides to activate some of the main features of a Genesis League Goals account, such as the ability to start earning rewards for their play by upgrading their account. Upon upgrade activation, you should be able to enter a new username to replace the default username that an account had initially been given. This username change would then become irreversible, as this action also opens a static Hive address, which doubles as both a deposit address for receiving and storing Hive assets, which includes Genesis League Goals assets, and your permanent username.
Your Hive address cannot be changed, though can be utilized as a username along with your private keys throughout various Hive websites and applications.