In my case, this version of "The Past" would be going through just the 2.0.7 levels, including its Match, CTF, and Circuit maps, with no earlier or later versions included. If we wanted to go back many versions earlier, we would have different WADS that represent each version.
Each map would replace it's modern version from 2.1, so you'd still play CEZ3 despite it not existing in 2.0, and you would have to battle Black Eggman twice, on ERZ3 and ERCZ (unless a flag is inserted to end the game at ERZ3, but personally I don't expect it.).
All the NIGHTS special stages would return to being the "Get ### Rings!"-styled stages, though if the coding for 2.0's special stages don't exist, we could edit the 2.0 Special Stages so they're played in NIGHTS mode, but with standard platforming controls as opposed to the NIGHTS controls. All rings become blue spheres, and somewhere in the level both an Ideya Capture and a "Goal" area are placed, so once the number of spheres needed is reached, a player destroys the Capture, takes the Emerald, and heads to the goal. However if such a feat is impossible, I'm fine either way.