So this strategy is working as suggested - thanks to all who replied. I eUpgraded my domestic/transborder Latitude Flight Pass booking at the time of reservation, then called and cancelled within the 48 hours of departure window, and went online to reserve a complimentary Business Class seat.
I was worried initially, as the availability at T-48 was J0. I waited a couple of hours, checked EF, and found that it was now J2, so went ahead and cancelled/reacquired the upgrade (switching eUpgrade to complimentary upgrade).
Has anyone tried this when the flight shows J0?