Just to report, am in DXB again on the way to DFW, two of us travelling in J. I was watching ExpertFlyer and KVS starting around 3 hours prior to departure.
The ExpertFlyer seat map showed Y and J completely full and 5 seats left in F. KVS was reporting P4, J2, and Y7.
About 2.5 hours prior to departure I noticed on ExpertFlyer 2 seats in F got taken, leaving 3 left. J was still showing full.
I decided to head down to the gate an hour before boarding time and waited there about 20 minutes for the gate to open. I went in and asked the gate agent about the possibility of an op-up and whether it would be best to come into the gate early or later. He tapped on his keyboard and said "You two are already upgraded to F.".
Sure enough, the two seats I saw get filled on ExpertFlyer 2.5 hours before departure were ours! And that was with KVS still showing availability in the J and Y cabins.
The gate agent told me the flight was full and that we were Gold, thus the reason for the upgrade. However, the J cabin has nearly been full for several days, I would assume there are many Golds in the J cabin.
I did ask about the possibility of getting the window seat that ExpertFlyer showed as available, but the gate agent stated F is full, so it sounds like the other 3 seats got filled too. And sure enough ExpertFlyer is now showing the entire plane as full.
So, 2 for 2 now on DXB -> DFW op-ups. 0 for 1 on DFW -> DXB. The difference being the DFW-DXB flight had many empty seats, both DXB->DFW flights have been packed.