I’m in the same boat. I have AA economy booked to LHR and want to upgrade with miles and cash. I, too, noticed that only BA seemed to have seats available for upgrade, not AA operated flights. I’m waiting till closer to the date to (7 days prior) see if seat become available for upgrade. It does make sense that they would wait closer to the travel date to make them available as it is more lucrative to sell J outright than allow upgrades for relatively paltry cash amounts.