I agree with Dave on replacing the old piton and restoring the route to its original condition unless there is an obvious gear placement availabe (camming devices probably weren't around when many pitons where placed).
However I reckon old pitons should not be replaced with bolts. At least you can see the overall condition that the piton is in (I would trust a well placed piton over a bolt anyday) and if the piton cant be trusted then at least I know.
It would also be a helluva job lugging a drill up some the old country routes
and then replacing the bad bolt every decade and a half