The M and P type bars are swappable. The brake cables are the same, but the gear cables will need to be changed. The S type bars are not - they require a different stem; as you mentioned it's more curved to give a longer effective top tube.

EDIT: Oh, looks like the brake levers are different for the M and P/S types. Bummer.

I don't think it would be trivial to fit, say, the M type bars to an S type stem and still retain the fold, the bars would hit the ground before they engaged the handlebar catch.