I have tried everything from the debug room, to putting in inproper discs at the incorrect time to see if I could get ahead early in the game.
Anytime you save, and you turn off the game, and you reinsert the same disc you were on, it will ask you to place in the proceeding disc as soon as the title screen appears.
Lets say you finished disc one, alright? Then you slip in disc three? It will ask you to place disc two in it's place. Thats how it works. I know you didn't ask that last part, but I thought I would tell you.