On most spinning and baitcasting reels used in freshwater, all the wear on your old line has occurred on the first 100 feet or so. Assuming a reel capacity of 100 yards, there's about 200 feet of line on your reel that's never even been wet. So instead of spending about $8 per spool of brand-new mono line for one spinning and one baitcasting reel, just reverse the line you already have.