For my recent SFO-JFK 1pm flight, booked in coach, and applied a BXP1. I noticed that for this specific flight they stopped selling coach a couple of times, including 4 days prior to departure. Business had 4 seats available, while F has 6 seats or so, both selling at $3250/3450 respectively.

My upgrade was cleared 48 hours prior to the flight, and at that time business was booked full according to the seatmap. Did not check whether they continued to sell seats in J or F.

30 minutes prior to boarding time, the gate agent called me on my cellphone, letting me know that business was oversold by 2, so they are going to op-up me to F.