It depends on where you book. If you book directly with Air Canada, you have 24 hours to c

If you do it online, you can do this: Open two different browsers. Open your​ existing booking in browser A (you will need your credit card #) and start the new booking in browser B. Make your new booking and, once you have a confirmation #, immediately cancel your earlier booking. Your credit card should be refunded within a few days.

If you book through a travel agent, it depends. Many have no grace period, some do.

ExpediaforTD gives you until midnight Pacific time of the day of booking to cancel (i.e. less than 24 hours). Expedia.com gives you, AFAIK, until midnight if the next business day (i.e. more than 24 hours), but you should double check.