Bit of a newbie question. I’m booked in BA F for SEA LHR JNB and my SEA LHR leg was canceled and rebooked to an earlier flight. The earlier flight is problematic for me for a couple of reasons. I would like to rebook the AS flight at 1650 YVR and beyond, but I don’t see it as a flight choice on the BA website. I am curious if this is an option and what I should know before I call. Booked through AMEX if that makes a difference.