I managed to get the J to F upgrade on the A340-6 HKG to JFK last year with one of those upgrade vouchers (I was given a few as compensation for a couple of sequential incidents where they handled things very poorly). I was monitoring the flight for 3 days and on morning of departure there were 3 empty seats in F and they still wouldn't do the upgrade without some fight. I was polite, but firm about using it. I had to show them that they still had 3 seats for sale and they dragged it out until about 30 minutes prior to departure. My suggestion is to know how many seats are left in F before checking in and check in early. When you go to the J lounge keep asking at the desk and be firm but polite that they need to live up to terms of their voucher. My gut feeling was that it was killing them to have to give the upgrade on that flight.

BTW - The flight went out with 2 empty seats in F.