AFAIK (and I could be wrong), it is earlier same day change. I always book the latest (or a later) flight and try for an earlier one.
I just did this YYJ-YVR-YYC on Aeroplan reward ticket. The connect for the earlier flight that I really wanted (and knew I could make) was not a legal connection so I booked the next one and phoned the concierge line to put me on standby (if not SE, I think AC should be able to do it) for the earlier one. Despite the YYJ-YVR flight being a bit late, I still made it to the gate where the agent had my boarding pass ready.
In your case, a later flight might fall into the "risky" category.