Getting a seat on AC
I'm United Gold, and if I buy an itinerary from UA that includes a trans-Atlantic leg on AC, when and how can I choose my seat? I need extra legroom in economy which seems to mean only exit rows, yes? I'd have to pay despite Gold status, is what it seems like. OK, fine (not really fine, but that's life), but when can I do that? As soon as I've reserved? I gather that I can choose a non-exit seat for free 24 hours before departure, but not before?