Had an op-up Y(saver)-J, JFK-DXB. F wasn't full so went ahead and asked the purser if I could use miles to upgrade to F. She said I can't since I'd already been upgraded but I told her that it was possible. She brought me her 'rule book' which had language like "It is not advisable to upgrade revenue passengers to First Class who have been upgraded to Business Class." Would be interesting to hear if anyone still managed an upgrade to F after an op-up to J.