Are you planning on purchasing a new, longer boom as well? A sail with a 12" longer foot will not fit on your existing boom. Having the larger sail cut to fit may be false economy. I would suggest you talk with a sailmaker to see if that would make financial sense. Sail modifications which involve major reconstruction of high load areas such as the clew and reef points can be very expensive. Detailed areas like batten pockets are also time consuming and costly. The entire length of the leech may have to be cut to prevent interference with the backstay and each batten pocket may have to be adjusted accordingly. Add the cost of modifications to the price of the used sail and see if it is still a worthwhile investment. Putting the cost aspect aside, the overall design of the sail will be a compromise as the draft will be further aft than it should be. That will cause increased weather helm.