Originally Posted by YOWgary

If a known weather event is approaching - like a hurricane, or a region-wide blizzard - AC will generally issue a travel advisory offering free changes on bookings to/from affected airports. So, for example, if Montreal's about to get hit with three feet of snow, they'd likely allow you to change a YVR-YUL for free, but not a YVR-YYC.

Generally speaking, the travel waiver's only going to apply to the affected flight - so, for example, if your YUL-CUN got delayed by a day, no, they're probably not also going to give you a free change to your CUN-YUL a week later.