Actually - I can go one better - if its possible book a
fully refundable fare to the same destination on another carrier (ensuring a cash refund is possible upon cancellation on day of flight - which I think is the tough part), then normally you would cancel the fully refundable fare in the usual situation that QF does not downgrade you. But in the unlikely even that you do get an operational downgrade, whats to stop you flying the other carrier (i.e. JL or say SQ or CX) to your destination? Obviously calling your credit card company immediately and launching a disputed transaction "service not provided" for the amount of the QF fare? A sort of "self insurance" policy. Return flights could be structured the same way with the added bonus that an operational downgrade by QF on the return flight back into Australia would have QF in the hole for the entire return airfare if the disputed transaction sticks - I wonder if QF would be more willing to negotiate a fair outcome if they didn't have your money?
Now why do airlines oversell seats again??