If they gave it the open world instance treatment (like Eureka or Diadem) where you can change outside of combat then yes in theory is possible, but the duties via the Finder have been coded in a way to prevent switching whilst inside. I wouldnt mind if it allowed players to switch between anything within the same role i.e. PLD to DRK, WHM to SCH etc but not to something outside of those, so a tank couldn't switch to DPS. Otherwise tanks could just bypass queues (as it's quick for them) and switch to a non tank on entry in to the duty.