You may get sympathy for the involuntary schedule change by Aeroplan contacting Air Canada to opening up space on Air Canada flights but I think they are really only required to book you into available reward space. I have had it go both ways.

On a cancellation on Air India earlier this year, we were contacted proactively (2 weeks before travel) and secured on alternate flights with a different routing on the same day with one segment downgraded to economy. We were offered alternate dates without paying a change fee or collection of new taxes and fees for more stops. We decided to stick with the original travel dates. We received 2500 Aeroplan miles as compensation for the downgrade (if I recall correctly).

I have also been successful in getting them to open up business space when a schedule change violated a minimum connection time in LHR. I was able to get rid of a AC LHR-YYC-YVR and get the AC LHR-YVR direct in business by asking politely.

I suggest you have some alternatives in mind and ask for the flights that you want and hope for the best.