There seems to be 3 op-up location possibilities in general. At check-in. At the lounge. At the gate.

I think the main thing that determines where you will be checked in is probably the anticipated load-factor. ex-JNB they often op-up at check in when Y is heavily overbooked and J is wide open.

ex-DXB is probably a different kettle of fish because the load factors are probaby highly variable up until t-75 (and possibly beyond) not only because of no-shows, but also different incoming flights with connecting pax. Hence upgrades are more likely in the lounge or at the gate.

There is a fourth possibilitiy though. JED. probably unique. you still have a chance of being op-uped once your on-board. I have seen musical chairs on the tarmac a good few times :-)

If you have any EK status I think your chances of being denied check-in are fairly slim unless you actually try to check-in late.