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!

Worth a phone call, isn't it?..

If you are Gold and not that far from the Airports, or arriving on transit flight, then it may be worth doing it at check-in or the lounge - you never know, they may do a deal and let you go all the way for the same miles! Happened to me once, i offered on the CHC-SYD sector, I was booked in Y and SYD-DXB booked in J, they bumped me to F all the way from CHC-DXB via SYD and BKK.

Mine is similar; LGW-DXB-AKL is LGW-DXB-BNE-AKL. I currently have the option to upgrade from DXB-AKL, but not the specific DXB-BNE or BNE-AKL legs, they are shown as one leg, probably because the flight number is the same? Guess you'll have to do this over the phone rather than online, but I'd be interested to hear what happens.