If you lay any Aero sail flat on the ground you will see the Luff is straight for the 1st part up from the tack, then it curves to the head.
The curved part corresponds to the flex in the upper mast and is common to the 5, 7 , and 9 sails.
The straight part corresponds to the lower mast and is a different length for each of the sail sizes, and corresponds to the different lengths of each lower mast. The lower masts don't flex.
Put a 7 sail on a 9 lower mast and the sail tries to put bend in sections of the lower mast that just won't; ergo it shouldn't set correctly. Put extreme kicker & outhaul on and the result could be permanent stretch & distortion of the sail.
