Originally Posted by
theKrafty1tar
This is actually not 100% correct-
Upgrading with miles or evouchers will not change mileage accrual. You will still earn miles in the originally booked booking class. (The booking class is changed- but to an award fare code: O, I, etc)
Fixed price or bid upgrades made in advance, before check in, will change the booking class and changes the mileage earning on the flight.
Thanks for making it even more clear (not only) to me.