Originally Posted by
84fiero
The call sign Air Force One isn't just theoretical, it is standard practice that any US Air Force aircraft carrying him operates under than call sign while the President is onboard.
This is done very precisely and literally. When President Nixon resigned, he left Andrews on AF1. While in flight, President Ford was sworn in as his replacement, and at that moment, the flight Nixon was on was re-assigned a new designation mid-flight, as the President was no longer on board.