There are certainly FTers who are more expert than I, but I think if you are booked in P they would let you stand by for J seats on the 3 class aircraft.
In practice, it would be almost impossible to use miles to upgrade to F once you had a confirmed seat in J in such a situation. By the time they confirmed your seat in J, others would have been upgraded to F. Even if F seats were still empty the GAs would probably not be willing to take the time to process the upgrade for you.
In very, very few exceptional cases, a GA might just give you an op-up in such a situation, but I would put your chances at 5% or so.
Be happy that you are probably moving from F in a 757 to J on a 777, which is a major upgrade in itself.