To my knowledge this has always been standard practice in QF. Although they may reserve the offers to the more frequent flyers (or those with flexible tickets) depending upon the number of seats available on the earlier flight. That would appear to be a purely sensible commercial protocol. Furthermore, if they know a later flight is likely to be delayed or cancelled, they will "flow forward" as many passengers as they can to minimise disruption.
But buying a bargain-basement non-flexible ticket and then turning up early at the airport and "asking" for an earlier flight and then getting upset when they ask for the flight-change fee that was clearly accepted in the terms of the cheap ticket is a bit rich.