Has it always been prohibited to unequip and equip items from a hero's backpack, while it is not your turn?
In any case, the behaviour upon a player's attempt seems quite buggy:
In one game, when I tried to change an item with another form the same hero's backpack while it is the enemy team's turn, I get a message that only upgrading cities is allowed. So far so good.
However, the hero dialog nevertheless shows that the items were successfully swapped, but then synchronisation seems to fail. The browser asks me whether I really want to leave the webpage (no I don't) and I get the Request Retry window showing the red/green connection timeout bars - however I am on a stable connection and I only get this problem after changing the items off turn.
So apparently the server rejects the item swap, while the client allows it and retries to push it onto the server. So the only option is then to reload the client (and the items are then unswapped as before).
EDIT-PS: In the end I get the error "updates to fight order not allowed"