The T&C of the bidding offer require that you register a credit card and if your bid is accepted, the card is charged and there's no option to change your mind. But I'm sure there are chain upgrades as you describe.
My cruise is two weeks away and I have not had an answer from Cunard on my upgrade bid. I don't expect an upgrade, as QG has been showing sold out for a while and there's no way for QG passengers to upgrade to give me a place. I'm surprised my bid is still on the table so late. When I did an upgrade bid on QV last fall, I think the "sorry, no upgrade for you" email came more like 3 weeks out, maybe more. Waiting so long to place passengers probably contributed to the overbooking situation.