Sails made from 'flat panels' are 'broad seamed' so that the '3D curvature' occurs at a maximum at whats known as the Position of Max. Draft (POMD) or where the amount of draft is at a maximum in reference 'back from the luff' dimension.

Yes, its relatively easy to 'slash' off a foot or leech; but, if the point of max. draft in the larger but 'slashed' sail is now 'further aft' than the sail you are replacing is much further aft than the original .... you will wind up with a very cranky and more 'weather helm' producing sail. So therefore, each sail should be fairly 'close' to the boom length (the boat's "E" dimension) or LP dimension ("J" dimension) of each BOAT ... as these dimensions determine the approx. position of max. draft (POMB). If the two sails are close to these E or J dimension then you can 'shift' the POMD back and forth by halyard/cunningham tension ... but there is a limit of how much tension you can apply and thats the reason why I recommend that the sails from different boats be very carefully compared ... especially the BOAT's E & J dimensions as THAT is what determines the approx. POMB or where the maximum amount of 'camber' occurs.