I'm confused. Is your SEA-LAX a continuation of the ticket you have booked from the state of Alaska? If so, I'd call Alaska and ask what is costs to change to an earlier SEA-LAX. You really need to confirm something, versus standing by at the gate for a seat, so that your bags go with you. If you miss the Qantas flight, and are flying on separate tickets, you're pretty much at their mercy if you miss the flight. I would not want to be in that situation as you could end up not flying at all.

Another option would be to overnight at LAX and pay Qantas a change fee to push your Australia flight back one day.

Now, if Qantas sold you the ticket that includes the SEA-LAX segment, it should be easy enough to change by phone. There are "minimum connecting times" from domestic to international and I doubt 40 minutes falls within the acceptable range when a terminal change is involved.