You have a higher level key on 3 other toons(say 50) and sell all the other keys/hide them on other characters. Then the main goes and starts the lvl 40 grift and since the others only have a key above the rank your doing it will use that one. Main upgrades gems(making sure it keeps 1 for rerolling) and then change the toon who is creating the grift(buy back key from vendor on that toon)