Update...

Thanks all for your help. I ended up phoning back as I finally had time to call and wait on hold. This time the agent told me that the extra charge was for the nav can fees as suggested by highflyer88.

In the end, I spoke to a supervisor and (after being put on hold a few times) was refunded the money. The refund was because the additional charges were not disclosed to me at the time of the change (not only that but because I was specifically told there would be no charges).

The supervisor explained to me for futher knowledge that whenever there is a change to an Aeroplan ticket (including upgrading the cabin of a single leg, regardless of whether I had paid full points or not), Aeroplan must re-issue the ticket and that the customer must pay all of the applicable fees/taxes in effect at the time of the change.

To me this still seems silly. I could understand this if I had paid for an economy ticket, and was trying to upgrade the whole trip to business, but in this case I paid in full for business right from the start. It seems like some sort of IT limitation. AFAIK, if I purchase a ticket from AC with the plan to use a cert to upgrade, my ticket isn't re-issued when the upgrade is confirmed (maybe I'm wrong on this point), so why does Aeroplan have to reissue a ticket on what I essentially view as a change in seat assignment (the upgraded itin had a different ticket number than the original itin, but same locator number).