Well, a hole saw is out of the question…youll never be able to keep it centered. One option would be to mask the cutting area with tape to avoid splintering the glass, and use a jig saw with a very fine toothed blade. You could fill the old screw holes with either silicone or some of the putty like epoxy that you knead together, then fill the holes…its the consistency of play dough, and hardens quickly.