If you get enough advance notice of free changes by AC, 24hrs before departure, then call Qatar. It's a QR issued ticket, your contract is with them, they need to reissue.
Once you get to 24 hrs before departure the ticket now goes under airport control. It's still a Qatar ticket and their responsibility to change it but if you can get to the airport then you will probably find the airport staff have much more empowerment to change the ticket. Call centre staff often frequently struggle to do sensible things with tickets that are under airport control.
The chance that Air Canada call centres will be able to do anything useful is small - they would always have to refer to Qatar.