Thanks for the advice.

I've contacted the TA, asked and gave them the Qatar Promo Code, but they advised the price is still much higher than expected (i.e. I expect the difference for upgrade for all 4 of us would be around $6.5k, but they quoted close to $9k) No change fee, just the price differential + extra taxes.
So given I cannot contact Qatar Reservation direct about it, I think I will give up the upgrade option, and stick to Class N.
Cheers