I recently did that on an overbooked SFO-ORD flight. Was rerouted via DEN on a flight that departed 15 minutes earlier than the ORD flight. The ORD flight was changed from an 747 to a 777 to a 77Q, so they knew they were grossly overbooked, and were printing out the VDBs on the spot, no need to wait til boarding was over to see how many seats were left. Also helped the flights were only two gates removed from one another.