I have received these offers before. Some I take up and some I don't.

People get these offers generally when an aircraft is VERY oversold in economy and it appears that operational upgrades may be necessary.

Points upgrades would normally be ahead of any operational upgrades, these offers are an effort by Qantas to increase the effective revenue for a flight (by reducing QFF Points balances).

One offer I received was about a week before the flight. I was in O class, but the flight had been showing J9 ... Y0 ... for several weeks; I did not accept the offer and was op-upgraded upon check-in. Being a WP, an op-up that was almost a "Lay Down Misère". (I sat in Business class adjacent to two QPNB's who had been called up on the lounge.)

The couple of other times this has happened to me, the loadings have not been as clear-cut and I accepted the offer, each time the upgrade came through.