In Gears of War, if you replay an act on a higher difficulty for a weird reason like COOP or boredom, it saves that you have done this.

Example: I beat 2 on Hard. Later on, go though game on Hard, Just play 1,3-5, get achievement for hard.

Does anyone know if MP3 does this? I messed up my save, it has the chapter's unlocked, but I can't continue from that point, and repeating the last chapter unlocked by itself doesn't unlock the next chapter, so I'd like to blaze though the game on Normal and then once I get to the place I left off, switch to hard.

I thought you can continue from a new checkpoint.

Unfortunately I am not aware if the achievements stack.