You can modify your ticket reservation without paying a fee under the new Air Canada flight modification fee policy for 2021:
Regardless of your fare type, changes must be made within 24 hours of your reservation.
if you have a ticket that was purchased on or before April 30, 2021, and you intend to travel by that date.
if your original departure date was between March 1, 2020, and May 31, 2022, and you purchased your ticket on or before May 31, 2021.
If the following fares are altered:
Comfy Rates (60-plus days before departure).
Distance Fares (2-plus hours before departure).
Flexible Premium Economy Fares (2-plus before departure).
Flexible Business Class Fares (2-plus before departure).
Prior to departure, Award Fares 2+ (only Altitude Super Elite 100k members).