I’m not talking about overbooking and the need for VDB/IDB at the time of check-in or boarding. I’m talking about a situation like seadog83 is describing, by which seats on a particular flight get scarce enough well before the flight date that it would be possible to allow some passengers booked on it to shift to another, less in-demand flight, in exchange for some kind of compensation. In my view, this would allow the airlines to (a) increase their revenue by selling a more expensive ticket for the same seat while presumably paying less in compensation than the difference in ticket price, and (b) lower the need for VDB/IDB at the gate along with its messiness and general hassle.