Air Force One is any aircraft they choose to use, if he's on the VC-32A (B757), that's "Air Force One", it's a callsign used when POTUS is on board. The option is there.
Minor clarification - it is any Air Force aircraft they choose to use.
If it's a navy aircraft, it's Marine One.
He could still choose to fly on his private 757, in which case it would use the callsign "Executive One", as would any other civilian aircraft carrying the president.