$600 + 30,000 miles upgrades on transpac flights are generally good for all segments that takes you from point A to point B. In fact you are lucky that you can get upgrades on all segments. Sometime your upgrade only clears on one segment but the $$$ and miles are not refundable (as long as your get upgraded on the transpac segment).

For example, I've used the same $600 + 30,000 miles for LAX --> NRT --> HKG but only got the upgrade for the LAX --> NRT segment (the segment that really matters). There is no refund of $$$ or miles.

I am quite positive that there are no ways to pay a lesser amount of $$$ and miles to upgrade only a selected segment. I'm sure that someone who is more experienced in transpac flights will chime in.