Once the SWU is applied to the BA flight, it will revert to the BA Prime number. My AA codeshare in fare class P changed to a BA Prime flight number in fare class T; but upgraded to U. You should earn on the underlying fare class purchased based on distance.

I flew CPT-LHR-PHX in PE last September before SWUs could be used on BA. My "return" was PHX-LHR-BUD on March 6th, both AA codeshares on BA metal. I applied a SWU and was upgraded on both segments. For $1047, I thought it was a great deal.

I also purchased another ticket for my partner for just PHX-LHR-BUD, all BA metal but both AA codeshares. A SWU upgraded both his flight too so I don't think there is a strict requirement that a segment is flown on AA metal. It appears as long as it is AA marketed, you are good to go.

In November, I had some expiring flight credit I needed to use. The flight was originally transacted in CAD dollars. I found a YVR-DFW-IAH-LHR flight, BA metal, AA codeshare on the LHR flight in PE for $688 USD. I booked that & the BA flight was upgraded with a SWU. C space opened on an earlier AA DFW-IAH flight. That was an easy change and upgrade. Unfortunately, the YVR-DFW flight remained waitlisted and I didn't clear on that segment.

In my love / hate relationship with AA, I do appreciate the plethora of Y-UP fares they offer.

James