Could this be done with a global variable? For example, have a global variable switch to 1 instead of 0 at the end of your last scene in your first level of difficulty.
You'd have to figure out how to store it. You might look into how to save and load games.
I'm doing this as well, and I plan to do this via global variables. I'll work on this today or tomorrow and see what I come up with.
Hope it helps!