ACV bookings come with certain restrictions that regular AC bookings don't have. One problem I have had in the past is that in Irrops situations you have to call ACV reservations instead of the AC Altitude line and the ACV reservations closes at 9pm and doesn't open until 8am. I had a major problem with this last year at YYZ because my flight from NAS arrived late and I missed the last flight to YHZ. No airport agents would assist me and said I had to contact ACV reservations who were closed. Also to select seats you must call ACV and it cannot be changed online until OLCI. If you were to attempt to eupgrade you cannot do it online either. All changes to the booking must be done by calling ACV.