Recently I had to call about a schedule change on an AA TATL connecting to BA. I had already been upgraded on the AA segment with a SWU (expired in June). I didn’t even ask about the connecting flight as I was afraid that it might mess things up because the itinerary was way complicated and had a price change. The kind agent fixed my flights but then said my connection had space available and would get me upgraded. In the process they dropped a leg charged me more for the flights put me on hold for 1.5 hours, then took my number to call me back 2 more hours later fixed it and gave me my money back.
Moral of the story the BA upgrades are new and sometimes you need to HUCA.