Now that I've learned my GPUs won't be valid when I intend to travel, I guess part of me is now looking at Cash + Miles as means of upgrading.

In that case, does fare class matter? Is it only in terms of Priority? Can you buy the lowest Economy fare class and then just use Miles + Cash to upgrade? Does the Cash portion vary based on your fare class? What would your recommendation be for SFO to PPT to go to Bora Bora. I really want to surprise my Mum with Business Class but unfortunately my GPUs will no longer be valid then. Trying to explore other options!