@algorithmshark: This is possible, really old, had been done before.
Keep in mind that in your setting you'll have something like a floating door, which looks bad.
@Notgoodwithusernames: If only the levels were changed and nothing else old savestates should work if you just enter a level and die once (although it might simply work right away). As you only give the player 1 life that is troublesome, unless you manage to get some more lives.
Also; why are you stopping with changes? So far the changes are a hundred times better than the original levels, I'd recommend you try to change a few more sometime.
Also this shouldn't be harder than walljumping, which is
just doable without slowdown, under the right circumstances.
Of course you should fight fire with fire, you should fight everything with fire.
Oooh look at my userbar!
Sadistic Designer - testing Pit without tools.