Here’s an interesting scenario where I believe the BA carrier-imposed fees make absolutely no common sense when converting from a AA codeshare to a BA prime flight on a ticket associated with an instant upgrade.

I originally booked JFK-LHR-DUB in economy given how cheap the fare was. The SWU to J was looking dicey so I took the very competitive instant upgrade to J and confirmed into F with the SWU. When applying the SWU to the BA leg (U space was open), I was told that there would be ~$1k in BA taxes (carrier-imposed surcharges) due. I had this confirmed twice.

My theory is that because I took the instant upgrade to J, my underlying fare remained in economy without the various surcharges in place on premium tickets within the JV / the instant upgrade doesn’t collect those surcharges. The switch to BA prime was forcing a collection of those fees.

Have those that applied a Y->J SWU on an AA TATL segment run into something similar when trying to clear the short-haul segment?