Once. I was the default which was an goofy-looking idiot character (intended to go with my "dullard" alias). The goofy character looked somewhat like my current avatar. But a couple years ago Anandtech changed many of the avatars, added many more, and they deleted one avatar - the one I used. They replaced it with the now infamous black man avatar. So I was forced to switch to the closest avatar I could find.
I'm curious though, why does your thread summary only mention ATOT? You can't change it solely on ATOT and not any of the other Anandtech forums.