I have a basic question about upgrades on AC. I just moved to Canada and do not know the FF program well yet.

I am currently in standby for a upgrade on AC848 (YYZ-LHR) this Saturday. When trying to access the seatplan using AC's iphone app to check whether a better seat would be available in economy in case I do not get an upgrade, the app took me to the business cabin. I see 21 unassigned seats out of 42. Seems too good to be true... Is there a known glitch with the iphone app? In cases like this, does AC assign seats to waitlisted people before boarding or do they usually wait?