who knows what call sign it would go under, probably “Air force I” because it was on a dedicated presidential role.
If the spare aircraft is needed to step in for the assigned aircraft it will be referred to as SAM (Not REACH as that's an Air Mobility Command callsign) 28000 or SAM 29000 until such time as the President steps aboard.
Back in 1974 when Nixon resigned he was being flown home on Air Force One for the last time and shortly after the appointed time when his letter of resignation was read by Henry Kissenger the crew of Air Force One made the following request of ATC.
"Kansas city this is former Air Force One, please change our call sign to SAM27000."
The man onboard was no longer president and the aircraft, mid-flight, was therefore no longer deemed "Air Force One."
So you see they do take the use of the call sign quite seriously.