I read in a magazine several years ago that on American, specifically, the agent boarding you as a standby on the earlier flight needs to do something to prevent the rest of your itinerary from being cancelled automatically when you do not show up for your originally scheduled flight. This manual move by the agent may be the move that wipes out your original reservation.

Once you are actually on the earlier flight, theoretically you have no further need or use for your originally scheduled flight. One possible consequence is that should there be a seating problem such as someone occupying more than one seat, the airline may have to both handle VDB's and have taken a standby (you) on the same flight. Going further off on a tangent, it is not desirable to volunteer after a successful standby since the gate agent might re-identify you as a standby and not compensate you.