Prior to the day of departure, whether there are upgrade inventory is irrelevant to your situation. The flight might be showing J9C9 but no D class is released. I highly doubt any D class will be released on this route if there are only 12 unassigned J seats as you said. Your best bet is to keep checking but you'll most likely get the seat if there are open seats on day of departure. This route is always pretty booked up in J.