Air Force One is the *call sign* of whatever aircraft the US President travels in. Once he boards another aircraft, the call sign AF1 is transferred to that aircraft (for a dramatic example of this, see the climax of [Air Force One](https://en.wikipedia.org/wiki/Air_Force_One_%28film%29) movie).

As it is not fixed to any one aircraft, if he flies in his 757, it will be called AF1. 

The VC-25s are USAF assets (operated by 89 AW) and are highly customised and specifically configured for this, costing over $300 million. I doubt any president is going to get the option of flying their favorite aircraft instead of the state aircraft.

Nobody is going to configure another aircraft for the president (The replacement for VC-25 is supposedly the 747-8). After all, no US President's personal car has been configured as the [Presidential State Car](https://en.wikipedia.org/wiki/Presidential_state_car_%28United_States%29) so far.