Originally Posted by Nile_US

Just as a data point:

I am MPC Silver and AA gold. I had booked my flights via cathay. I decided to bank those on AA. So, during checking, I requested to put AA FF number instead of MPC number. I checked in inside, at the gate, as I had flown domestically and didn't want to out and do the security again. A while later, they were able to print me the boarding pass and I was upgraded from PE(Y) to Business class (J?).

My flight was LAX-HKG.