How are you trying to pull it up? If you gave your aeroplan number, I find that I can sometimes pull up a reservation that I cannot get by using the reservation number (times when, like you, the flight number has changed). Then, you end up seeing that you have two flights booked (old and new number), until they've finished going through the system and clearing things up.
I've never paid for a premium seat, so I'm not sure how good they are at carrying them over, so calling is probably a good idea - especially if there was a change in aircraft type.
As a (potential) bonus - if this wasn't actually the flight time you wanted (but you choose it because it was cheaper), you can often get them to waive the change fees and flight difference costs, to get on another flight that day.
