I had the same change, along with a bunch of others through LAX. Our AA PNR still shows us on AA100 with seating reservations though - so not sure what's going on. According to EF all of the BA codeshares except for AA106 are gone, so that's likely what prompted it. Hopefully they'll restore them in a few days and I can get the bookings cleaned up.
Oddly though BA added an additional DFW stop before our LAX-JFK Transcon - extra 40TP & some Hard Eight BBQ at the cost of QF's S&P squid? Hmmm... 🤔