Not sure I agree. It's best done, politely, before the arriving pax have settled in. Cabin crew are terribly busy during loading and if you ask one, even in F, chances are that they wont be able to do anything till the pax in q has already stowed shows, unstowed cosmetics books etc, started the video and so on. No, I think it is best done politely and sensitively in person .. and if there is an option then offer the other guy the upgrade! (e.g. if you are seated in 64A and 63B then offer up your 64A for their 63A). Just courtesy...