USAF inserts plan for AF One replacement into future
The current Air Force One (all two of them) entered service in 1990 and 1991 and are based on the Boeing 747-200. The time appears nearing for a replacement.
Defense News reports that the USAF has added replacements in its future planning.
Given the long history of the Secret Service demanding more than two engines, a replacement would almost certainly be the Boeing 747-8I.