I've done this and it's a stopgap for four or five seasons at best. Cost to me: $300 to go from a 1984 Dacron main in sad shape to a three-year-old Kevlar-Mylar composite that fits closely. It's deteriorating much faster than the Dacron would, however, and will buy me at best another season or two.
I have had better luck with a No. 1 and No. 3 recut racing sails. In the future, I would pop for a new main cut exactly for my boat ($2,600-$3,000) and keep on with the recycled foresails, which are plentiful in my boat's foretriangle dimensions from various pickier-than-me racing types. Cost to convert (cut and change from tape luff to hank-on): $130-$150 per headsail.