Originally Posted by Guy Betsy
CX would calculate the cost difference between the I class fare and the full J fare.. and then the difference between that and the full F class fare.
Basically if you want to be sure that you get to sit in F, you have to pay or upgrade using Asiamiles to sit in F.
That seems too convoluted even for an airline. I would have to see the fare rules on that specific fare basis, but I would guess that even the rate desk would just figure the difference between I and F.