First of all, thank you very much, I'm glad you enjoyed it!
And about the stage changing? You mean the 3rd and 4th bit? Those shift the level half a length respectively right or left.
And all the levels exist on the same "map", so you can sometimes see them peeking through, and for some levels we designed it so that there's a little bit more of the level there.
I hope that clears it up a bit :)