Had a flight booked on AA from SFO-SEA-LHR. The SFO-SEA was AA codeshare operated by AS. AA had a change to remove most of the codeshares from the day of travel, but AS is still flying the original flight. Now that AA has changed the schedule they are only offering changes to another codeshare flight which would mean either 12 hours layover or requirement to rebook SEA-LHR on a different day and lose the upgrade. Asked if they could just keep me on the original flight even though it isn't a codeshare anymore and agent said no. Is AA policy only to rebook on codeshare flights even if the originally booked flight on AS didnt change at all?