I've seen DL17 LAX-SYD go out multiple times before the ATL-LAX DL17 even lands.

If you HAVE to be on that LAX-SYD flight, get an earlier flight to LAX (3 hours, give or take, at least).

Another item to remember if you upgraded that flight using PMU or other ways you still PURCHASED a coach seat, and depending on inventory for the next flight you may not sit upfront. That's one bugger of a flight to be in the back, especially if you get a middle with two, ehem, fat guys, on either side of ya.

There was another thread about this same thing posted up last week or so where others chimed in. Look it up.