I did this in April for AMS-EWR and EWR-AMS for my wife and myself. Called the FD Elite desk in Amsterdam to request this upgrade (about 4 weeks before the travel dates) - took some time. But when my travel agent emailed me the ticket numbers (they had already told me those by phone, so I could request my upgrade) a few hours later that same day, she already saw us confirmed in BF. Called FD desk the next day, they confirmed it too and faxed the vouchers. No hickups at all - still amazed at how easy this was. Tried it on NW flights a few times, but it was always a problem, or turned out wrong. If I had to do it again, I would again call the FD desk to start the process.
Good lucck,