The same holds true with the vice president. Any aircraft can be designated as Air force 2 that the VP is on. My son flies (commercial line) out of Regan in Washington DC and sees many high profile people flying on his aircraft. Once Al Gore was on his plane and they were number 43 in the queue for takeoff (a LOOOONG wait), the secret service did not want his plane stuck in one spot for an extended length of time for security reasons so they approached the cockpit and told them to tell the tower they were changing call sign to Air force 2. The tower told them to taxi using the RUNWAY since the taxiways were full of waiting planes and took off. RANK HAS PRIVLAGES! Also, just because the VP is onboard a commercial plane it does not HAVE TO change call sign if not deemed necessary by VP's security staff. Many times the VP flies without many people being aware.

When senators and other high profile people fly they "Block" seats (empty seats) all the way around that person and if VP the "block" is larger and Secret Service occupies the blocked seats and an air marshal is aboard. I believe ALL flights leaving Reagan HAVE air marshals aboard, if I can recall.

Another change in call sign occurs when a human organ for transplant is aboard. The call sign is the same but with "Life Flight" added to the end. First priority is given on take-off, landings, and air space in traffic. The crew loves this! Better than VALET PARKING! LOL