Resheaving Main Halyard
Something else you might try before lowering the mast is a round lead weight on some fishing line. Drop it down and pull the line up. Lean the boat to the side where the halyard exits, and pull all other halyards taunt. When you tape the new halyard to the fishing line, taper it out quite a ways and lubricate the tape with dish soap. This works nine out of ten times.