Yes, but as stated before, they can only rebook you for free if the agent sees availability in the same fare bucket, and, as mentioned, other fare bucket seats are not "released" for stand-bys until ~ 3 hours before flight time.

I did this recently, and was advided by the person on the elite line to:

CANCEL my reservation for a 9:30 AM flight, call up 3 hours before the redeye flight I wanted shortly after midnight. Since my flight itinerary was "cancelled", I had to call in and give my conf. # and it took them a second to "find" my old res. and re-book me as a standby.

The good news? I still got upgrades on both legs home...