Last time I had to call into reservations I was ready to pay a change fee to move to an earlier YYZ-YVR flight (equipment downgauge and the flight I wanted had R space within my upgrade window). The res agent managed to do the change for free as my fare class was available on the new flight and it was to give me extra connecting time in YVR.

However, that ticket was not booked with an OTA. Any changes on tickets booked with an OTA, AC has told me to talk to the OTA.