Originally Posted by
natcin
i remember reading somewhere on FT that for those flights delayed with daily schedules in/out of an airport, they are renumbered to prevent confusion with the next day's flight.
Yes, that's the reason. They used to add a letter after the flight number instead of attributing a completely different number (e.g. AF023A), but this letter was incompatible with DL systems, so for US flights, they now give a different number. They still use the letter system for non-US flights.