I'm considering a LAS-LHR flight via DFW, on WT+ (the outbound will be direct). The AA flight from LAS-DFW will therefore be in economy - I'm wondering if it is possible to upgrade to first class for this leg, after booking?
Only if the LAS-DFW is booked in fare class J, C, D, W, Y, or B.
See Upgrade using Avios on BA, AA and IB Flights on ba.com:
1. Upgrading using Avios Points bookings on BA, AA and IB Flights can only be made online at ba.com or direct with British Airways. An existing booking can be upgraded up until check in online at ba.com or by contacting your local service centre or sales office. Upgrading using Avios on BA, AA and IB Flights cannot be booked at airport ticket desks.
2. The upgrade is a single cabin upgrade from the cabin booked, to the next cabin, this may vary depending on which of BA, AA and IB or a franchise operates the flight.
3. Upgrading using Avios Points on BA, AA and IB Flights are permitted on eligible published fares only (Fare classes are: J, C, D, W, Y, B).
4.
Upgrades using Avios Points are not permitted on bookings made with a travel agent. Upgrades using Avios Points are not permitted where flights are sold as part of a package holiday or where flights and ground arrangements, such as prepaid hotels, cars, transfers and experiences, are booked and paid for in the same transaction. Flights booked and paid for separately from any ground arrangements remain eligible for Upgrades using Avios Points.
5. Upgrades using Avios Points can only be applied to bookings which are made in the member's Executive Club membership country.
6. When upgrading a combination of BA, AA and IB Flights the Upgrades using Avios Points on Partner Flights conditions of use apply.
7. For upgrades on British Airways flights only the Upgrades using Avios Points on British Airways Conditions of Use apply.
8. BA, AA and IB Flights or codeshare flights are eligible.
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http://www.britishairways.com/travel...upgrade_baaaib