IME it’s a mixed bag of possibilities. The only consistent thing is whether QF are expecting weather delays at MEL or SYD. Then they are very happy people ask to be moved forward.
Yes from my experience especially on the Mel -Syd route where there is heaps of flights you also can get offers to move to an earlier flight. I had that happen on an award ticket. My wife had a paid ticket, on the same flight and was given the same offer.