Perhaps you thought that brave men balanced on catwalks high above Symphony Hall to replace lights in the giant chandeliers. Not so.The six chandeliers, weighing more than 1,000 pounds apiece, are lowered by electrical winches in the attic to keep from damaging the intricate wiring and hundreds of Austrian leaded crystals in each light. Besides the 24 visible bulbs, each chandelier has 76 interior 7.5 watt bulbs that keep the crystals sparkling. Cleaning, polishing and relamping has kept a crew of four busy for two full days.