Surely there are more substantive fixes to this problem. I understand that some US credit cards allow temporary numbers to be used for online purchases extra, and more than one of these could be active at the same time.
Even my 28 degrees card has a different 16 digit account number from the number on the card. This stuff is not unheard of, having multiple numbers in the system.
Since the system can cope with this, Qantas could issue each card with a "temporary" Qantas cash card number that lasts the lifetime of issue of that card. When they issue a new card due to status change, anniversary, etc, then it gets a new "temporary" number that does not invalidate the old one. The old one can then remain valid for the life time of the card, so if you started as a NB and went all the way to P1, you could still be using the bronze card to make transactions. Qantas could issue NB cards for two year expiries if Qantas cash is active on them also. Other cards typically only have a lifetime of less than 2 years anyway. (Don't know about lifetime PS, SG)
Then there would be no issues with any of the issues that people are experiencing here. The Qantas Cash product needs a product upgrade! Otherwise, it is simply not fit for purpose with this big gaping hole. I was thinking about using it, but not after this!!!!!
EDIT: And under this, if you have a pile of cards for going from NB to P1 in the same year, you could use any of them, whatever floats your boat - a series of different card numbers that all access the same Qantas Cash account. Easy - no problems. I see this as an essential fix to make the product a useful one.