Hello! If this takes place, then the EMT probably did not use the correct method. They should tilt the head while keeping the chin up. This prevents them from being able to gag. You would not want to change the oxygen flow because that one was probably correct for their situation. So just remove the air supply and follow the head tilt-chin lift procedure.
I hope this helped!
I am, yours most sincerely,
SuperHelperThingy