After the new site launched, I was curious if you would be able to change your username again, so I went and checked. As of now, it seems that you can change your username an unlimited amount of times, but it only affects the URL of your profile page. You also can't change it back.
For example, the URL is giantbomb.com/profile/Agnogenic but I'm still showing up as SpikeSpiegel everywhere else.
I hadn't seen this mentioned, so I just want to give everybody a heads up. Don't try to change your username!