An (interesting, to me) data point.

My wife and I are both AA PLT and were booked on BA/AA codeshare from MXP to LHR. A combination of Terminal 5 (shudder) and crew shortages cancelled the leg; and BA rebooked us on Swiss. Our friendly BA agent within the lounge said "You cannot come in the lounge. We have washed our hands of you"

While part of her phrasing was probably a poor translation on her part, I was pretty ripped. I called AA when I returned and my suspicion was confirmed -- since I was booked on a AA/BA ticket, even though they rebooked me on Swiss, my lounge access should have been intact.

Not sure if anyone else has had a similar experience. In fact, this is the only time I've ever been refused access to a lounge... and was quite a bummer.

Cheers,

-Andrew