We are not sailmakers but our understanding is that it is not a good idea to do that. You’ll be cutting a sail designed to hoist straight up a mast to one designed to hoist at the angle of your forestay. Cutting a main down like that, and getting the shape to work requires a great deal more work than you might expect. And by the time you’re done, it could will result in a sail where the shape and depth are still wrong. All without saving as much money as you might think.
That said, our advice is to ask a pro.