As Timetrain said, it wouldn't make sense for the Texaco station to be able to do that because Doc worked on the car independently, but it would be nice if you could change from the BTTF 1 car to the BTTF 2 car.
In addition to both of your thoughts, I also think that the BTTF 3 cars should be able to turn into the BTTF 2 version, too (via the Texaco garage). Even though it's not entirely movie accurate, it would make sense.
The last thing I have in mind goes a little off track from BTTF, but it would be cool if the Pay n' Sprays let you choose the color that you wanted for your car. It's kind of something like the Trans Fender garages in GTA SA. The only limitation would be that it would only let you select colors that are available for that car in the carcols.dat file. This way, if you drove a DeLorean in, you would only be able to pick "Silver."
Anyone agree?