Coming back to this with a similar situation and relating some anecdotal evidence to look for comment/similar experiences.

We have a flight booked with BA Holidays. All BA flight numbers. The initial leg is on AA LAX-LHR (we're getting from SJC-LAX on a separate AS ticket). We're in I. It shows up in the AA app as AA 108 but, of course, they know it's really ticketed as BA 1540 (I believe that's the number), so not possible to apply SWU to move into A at this time,

What I have been told, however, is that they can do this at the airport if there is still A available.Is this something generally true or likely an exception? Does anyone else have experience (yay or nay) for this situation?

Cheers.