I would never suspect that if I agreed to change my flight so that the airline could accommodate someone else that I would not receive any compensation...and I am sure I am not the only one. You are doing the airlines a favor - whether the flight you agree to change to at the last minute gets you there faster is not of any consequence. I have never heard of anyone being VBD'd and receiving nothing in return. (And I would not consider a food voucher any kind of compensation for changing my itinerary!)