Mélusine: Once any fellowship tournament starts (team challenge or site team tournament) - you can LEAVE that team at any time and join another fellowship's team. There is nothing keeping you on that team once the tournament starts.

I just tested it - I was in one fellowships for the Knight Fight team - I had already removed myself from that team (continuing to play the games for that tournament), and joined another team. Not sure about the issue when a fellowship is deleted, probable just a bug in programming, but for normal running fellowships, you shouldn't be trapped and should be free to join another team if you want.