Why? I've been in situations with a full J, seats in Y and F. They have no reason to roll people forward J->F in this case. They'll stop selling J as they tycpically do not oversell premium cabins. And, even if they did oversell J, the may roll some of thise forward to F, but still not move any Y->J.
Cheers.
I used the wrong terminology. Its true that they won't always op up people to fill J but on the NRT flights there is almost always more than enough Y passengers standing by for award or VIP upgrades to fill J at the gate. F will typically also fill up from last minute upgrades from J. I fly alot, see this all the time.