I just got off the phone with an Elite Desk agent asking about the same thing. I am schedule from OKC to CAE through IAH. There are other flights connecting through EWR and she said that I could stand-by for the earlier flights even with a different connecting city, she did recommend going through original city as, if the connecting flights were near capacity and not able to switch to earlier connecting flight then I would be out of luck in EWR (if unable to confirm both segments of the standby flights).