When you have two segments sharing the same flight number, and appearing as just one flight "with a stop" on MMB, can you upgrade just one part of that?

I'm flying from New Zealand to Dubai.
My itierary in MMB shows just AUK-DXB and DXB-CHC, but it's REALLY AUK-SYD-DXB and DXB-SYD-CHC.
MMB isn't letting me upgrade either of them, and flightexpert tells me this is because there aren't any D-class seats in the SYD-DXB segments. BUT it looks like there ARE D seats available on the AUK-SYD and SYD-CHC legs. But because they don't show up as different flights in MMB, MMB doesn't give me an option to upgrade.

If I call them, would they be able to process the upgrades on just those two shorter legs for me, even though they're not showing up as individual flights in MMB? Or would they tell me that if it shows up as one flight, on the same plane, that I can't split it up into its segments, inspite of the stopover?

Thanks!