Say for instance, you've booked a B fare to HKG and all domestic segments are already upgraded at purchase. Is it possible to do a *A upgrade using OnePass miles for 25k on the BF segment as opposed to the 30k standard award upgrade?

I realize you lose the ability to waitlist and would have to wait for inventory to open up, but if this is allowed you could potentially save 5k miles.

Anyone done this before?