This is existing wiring from the mid 60s on all the outlet boxes and all outlet boxes have a ground wire attached to the metal outlet box and they were originally 2 prong outlets and I want to convert to 3 prong do I need to use another ground wire and attach to metal box and green screw on the receptacle or is the 3 prong automatically ground by being attached to the grounded metal box. I also used the 3 prong tester that said it was grounded I assume they work. Thanks