Incorrect assumption
You were trying to compare an upgrade to selling a house, and it appears in the UK and Wales you can accept a higher bidder after an offer has been accepted but that's being ignored now
In fact, the Qantas Condition of Carriage are quite clear in regards to this:
4.7 Seating Selection and Allocation
Although we will try to accommodate your seating need or choice, we do not guarantee you any particular seat. We may need to change your seat at any time, even after you have boarded the aircraft, for operational, safety or security reasons.
If we need to ask you to downgrade for any reason, we will at your option:
- provide you with an appropriate refund of the difference in fares (or an appropriate credit of Qantas Points in the event that you are travelling on a Qantas Frequent Flyer Award), or
- accommodate you on a reasonable alternative available flight on our services."
You won't get an explanation, "operational reasons" is about all you will get, if anything
The bad part would have been the staff member at NRT not being that helpful!