Air Canada has and it is covered in the AC forum. I suspect all the airlines will be similar. So for 014 tickets the summary is.

1.) You can already change now and race to get back by 21-Feb - no charge/penalty and you can book discounted Y in buckets up to "M" while PE and J can book in any class.
2.) You can otherwise now make a change to your itinerary date - +/- 3 days from what you had originally for the flight into Canada.
3.) You can make a change now to add a "Stop" at the arrival airport and also rebook your expected connecting flight - I guess I would recommend taking the one at the tail end of the 3 days.
4.) If you get a negative test - you also can change for free one time in Canada to run to the airport and take the next flight as you wish.

So - before you go to Canada you can already plan your expected connection and then if you want or need to change that - you can do that after you arrive to Canada again for free (total of 2 free changes).