Thought I would post my experience doing a same day change with a routing change on an Aeroplan business award ticket. I was ultimately successful but it took a full hour and a lot of persistence, so might be informative for others.

Original itinerary (only flight available when booking):
YVR-YEG (I)
YEG-YYC (X)
YYC-YYZ (I)

Arrived at YVR priority check in, 2:30pm. Asked to do same day change.

Agent said OK at first, but then when she realized I was changing routing (asking for direct AC152, YVR-YYZ), said that this was only possible at ticketing desk. Not sure if this is true, but there was no line at ticketing, so no big deal.

Asked ticketing agent to do same thing. The agent that is supposed to know all about these types of changes. Obviously not exact quotes below, but close enough:

"Hi, I'm currently flying [blah blah blah] but I'd like to make a same day airport change to a direct flight on AC152"

"Oh, you have to call Aeroplan"

"No, it says right here that I can do a same day change at the airport on an Aeroplan reward ticket."

Agent printed out my entire fare rules (I think) on an ancient dot-matrix printer.

Agent then punched around on the computer for a few minutes.

"It's not possible, there are too many connections to make the change"

"Okay, is there a help desk or supervisor you can check with?"

"Hold on"

AGENT SPENDS LITERALLY 40 MINUTES ON HOLD TRYING TO REACH INTERNAL HELP DESK

Meanwhile, this is the only agent at the ticket desk. A large line formed behind me and a guy was freaking out because he was wanted to change his international flight and was going to miss the check-in deadline. Whatever, not my problem.

Finally, help desk answers and fairly quickly makes the change. Help desk tells agent to charge a $75 fee (correct) and I pay. Agent is about to let me go, but then:

(to the help desk) "Okay, the entire itinerary just disappeared"

A few more minutes of keyboard-tapping ensues, and I am ready to go on my direct flight.