I have been told by check in agents in both LHR and BOM that I could not use miles to upgrade from J as there was no availabilty in F. I checked in the lounge in BOM if the seat next to me was empty an hour before departure and was given a F class boarding pass there and then - for free! In LHR when we queued in the lounge to board the big bird, my name was called, they apoloigized it was full and I was given 2A - the best seat on the 380 as far as I am concerned. On both flights, miles were never deducted. I just got 2 nice comp upgrades, and they felt more comp than any others i had had before, or had since!