Originally Posted by
Andriyko
You will loose the ability to upgrade if you book the flight as a codeshare. The rules did change a while ago and codeshare flights are now excluded. You must book the flight under the prime number in order to upgrade.
As an added data point, this is also true if you end up getting rebooked/rerouted. If the original flight was the AA codeshare on BA metal, you'll still be ineligible even if you get rebooked onto a BA code / BA metal flight
