A minimum of 200 Quests is required to change your username. As an alternative, you can create a new account with the username you want and delete your old account with the Delete Account button.
You can change your username if you have 200 quests? I did not know quests had any other use besides just showing them off.(And, a fair bit of self-satisfaction.) Is it possible to find out what happens if you get more quests. There was nothing about it in the help section. Also, how come some people have armatars that are different from the available ones?