I want to sit next to my girlfriend for an upcoming AC flight. Will we be able to select the preferred seats for free during check-in if all the regular seats are assigned, past T-24 ?

If all regular seats are already taken, then they have to assign us some of the preferred. In that case, can I call past T-24 and request to be assigned adjacent seats by phone or do I really have to wait until we arrive at the airport to request adjacent seats, if left ?