And yes I do realise that everyone and their dog will ask for flow forward but so be it. Limit the requests to one or 2 flights forward to weed out those that book the 8:00pm cheapie and turn up at 8:00am.
The thing is though, a 9am MEL-SYD flight (which the OP was originally booked on) is often "a cheapie" compared to the early morning flights (which he was trying to change to). 9am is after the morning rush, so one way fares at that time can be $200+ cheaper than on the early morning flights.
Consequently, the OP might well have come across as someone who deliberately booked a cheap fare outside of peak times and then chanced his arm to try to get on a much more expensive earlier flight. Just to be clear, I'm not accusing the OP of doing that, I'm just saying that the QF staff might have suspected that to be the case.
Also, while I generally agree that empty seats are unlikely to get sold very close to take off, MEL-SYD early in the morning is one example where that probably happens quite a bit. I also know from experience that empty seats on those flights are a rarity. As such, I am not at all surprised that the OP's request was denied.
As others have said, the only cause for complaint that I can see here is that a BP was issued and then taken away. That is simply outrageous, and very strange indeed - it would be interesting to hear QF's side of the story, but obviously we won't.