Sorry for the resurrect, but I agree that current solution is far from ideal.
I want to do a few arena runs with my sorcerer but then I cannot level another class (without 250 wins) or use my main pvp team for the next 6 days. Hero should be tied to any team slot like everything else, and in the meantime the switch should be free of charge. Current option is just a trap.
As I have no choice to use a 4th troop instead of hero in arena, I just don't see how I can enjoy this mini game without having to degrade experience in other game modes - a pity, as I generally enjoy draft in games.