The ability to standby for an earlier flight is determined by the fare you hold. If your fare does not allow for standby, it's still worth asking, but be prepared to get a "no". Just make sure to keep your luggage with you until you know whether you can get on the earlier flight, in order to get it retagged (they will NOT allow you on the earlier flight if your luggage is on the later one). Incidentally, I've done exactly what the OP said many times, but always on fully changeable J tickets.