I just booked an itinerary on aa.com which includes the AY-operated HEL-ORD.

Usually, when I want to change seats on such a flight, I call AA customer service and ask for the AY PNR and then change the seat on the website (or call the customer service when website does not work, which is often enough).

In this case, I called AY directly (because AA customer service is busy), and they claimed that since April this year it has not been possible to book seats on flights which involve codeshares because of some system upgrades that are going on.

Has anybody heard anything about this? Is there a workaround? Can't say I am very thrilled with the idea of being unable to book seats on the longhauls...