If the old ones are mortise style (have been pocketed into the edge of the door), you can cut some blocking to fill in the mortise and redrill the door. You would then need to fill any screw holes, sand and repaint. Or, you can get a modernization kit that includes plates to cover all of the previous holes. Look for a Kwikset KW-214 or KW-215 mod kit. There are two sizes. You would drill the door to accomodate the new lock and mount everything using the mod kit.