Yes it can happen. I have seen it happen before on a BOS-LHR flight where Y was oversold and someone apparentely asked if they could move to a flight two days later in exchanged for being bumped. As they were boarding the GA asked for that volunteer to come come back to the podium as they could meet his request. Not sure if it will work on a return leg, but even if it did, you'd have to wait until the airport and see if they are actually oversold.