Just looking for some insight into a slightly complicated ticketing issue that's come up over the last few days.
Originally booked CUN-MIA-DUB (1st leg AA, 2nd leg EI on a BA 125 issued ticket) as an Avios award fare. Had to call to complete booking, as taxes needed to be calculated by fares team.
Came back as £156.10 + 24,500 Avios. No problem, booked, and etickets issued. This was slightly less than expected when I looked at both standalone taxes for both legs on the BA website - which came to in the region of £190.

Our plans have changed slightly, so I called BA earlier this week to change first leg to MEX-MIA, and continue to DUB as planned.
After a week waiting to get a fare quote, they've suggested the new tax is £275.10

For context the current calcualtion on ITA Matrix for our new proposed flights is as follows: MEX BA X/MIA BA DUB Q14.00 M 1948.00Y1N0C9S1 NUC 1962.00 END ROE 1.00 XT 8.60S1 50.40XD 70.40XO 5.50YC 5.50XY 3.00XA 4.40AY 209.40YQ 3.60XF MIA4.50

They have suggested my tax might have been underpaid on my first booking, which didn't include YQ. My confusion arises as I know EI do not charge a YQ segment/fuel surcharge for their transatlantic segments when booked via Avios or BA. This query is waiting with upper management what next steps are RE: payment of YQ regardless of whether I formally complete the change of booking.

Am I right to call out that no YQ should be charged based on EI doing the transatlantic leg, or does flying first leg on AA metal complicate this? Happy to provide more details but just very confused that this is even possible!