Rare that one would be upgraded on a VDB. Even rarer would be for AA to put you on another carrier-I would assume only when subsequent flights are sold out for the next few days. As others have said the software is very good now at predicting the number of no shows for any given flight for any given day and at some point AA might cease sales on a particular flight based upon the number of already over sells. The downside of volunteering is that you will need to wait towards the end of boarding at which time if you're told to board (the likely scenario) you might lose overhead space near you and possibly be forced to check your bags. Also if you've checked a bag that bag will likely go with that flight even if you don't.