it is just over 2.5 hours journey - I can't remember ever having to go through battles to get to the rear of the aeroplane

If I could get away an hour earlier on a 737, I would do it in economy.

Looking at the fare restrictions, there shouldn't be an issue changing to an earlier flight, though there could be a fare difference to pay if the earlier flight only has availability in a higher booking class

I would suggest that it might be better value to just book a cheaper discount premium economy or business class ticket on QF3 and wait the hour than pay the cost for a flexible economy ticket - or just pay under half the fare for a saver fare and just look at what the cost to change would be on the day. $307 fare + $125 change fee ( if same booking class available ) might be better than $757 up front