RE: Switching batteries in flight.
Unless you have an unusually large airborne pack your transmitter pack should outlast it many times over. However, your question is can you turn off your transmitter, replace the battery and then reacquire control of your aircraft, has this possible answer. If you have proper failsafe setup and are high enough so the plane can safely glide for the required time, then Yes, it should be theoretically possible as the plane would be in freeflight mode for that time. I would not trust doing so myself but, theoretically, it is possible.