I'm not encouraging anyone to abuse this if it is not supposed be how it is. I'm just trying to help out.
The way I accidentally figured out how to do this is:
1. Go into the Battle Subway.
2. Click 'My Account'.
3. Your Username is now editable.
Caution Changing your username also changes your login details.
The reason I think it's a problem is because when you click 'My Account' when you're viewing the "PokeBase" or "Meta-Base" the option to edit your Username is not available.