I doubt that would be permitted. One class upgrades using miles, whether Y-J or J-F are calculated from the base point of a normal flex or saver fare - as opposed to a ticket that has already been upgraded once by means of a specially discounted offer.

But ......even if I'm wrong on that, it seems a bit of a waste to be using miles to move from J to F on a flight lasting just 1.5 hours .....