Have the same problem with an alt account of mine. Tried everything I can think of, but nothing works. I can change it fine on my original account, so it's not a problem on my end. Does it tell you "you cannot edit an avatar during a duel" when you try to change it. That what it says for me. Problem started about a week ago I noticed shortly after I got disconnected during a duel, so my guess is it's related to that and other duels people got disconnected from lingering on in the arenas and not getting cleared away like they're supposed to when they're finished.