Afternoon all!

I have a Multi Carrier Award booked for March on CX, QF and BA LHR-HKG-TPE-HKG-PER/SIN-LHR.

It's a mix of First, Biz and PE, but priced as a Biz ticket on the award chart. Annoyingly the HKG-TPE-HKG legs got downgraded from Biz to First because of equipment changes, no avios or tax refunded (which sounds correct to me).

The PE leg is HKG-PER on CX for which Biz availability has opened up today.

I called up today to have that leg upgraded, and after a good 20 minutes on the phone because they couldn't see the availability I could see online (apparently the agent wasn't searching one world partners, not sure why!), they came back to confirm they could upgrade into business, and would offer to waive the additional avios (there wouldn't be any payable...) but that I would have to pay additional tax of ~£90.

I checked the tax amount by searching the redemption online as a standalone flight, it is £39.80 for Econ and PE, and £40.60 for Biz. All the other flights in my itinerary are First and Biz so it just doesn't seem to make sense that an 80p difference suddenly becomes £90... In case it's helpful, the entire taxes for the whole ticket were about £550.

Maybe I'm missing some logic here so would love to get your thoughts.