Originally Posted by KKILR3

Now it isn't the same site as you would use to select seats if you booked on aircanada.com directly. But if you obtain the AC identifier from United.com, then pull up your reservation using that code on aircanada.com, up at the top there will be a link for seat selection that says This booking appears to have been completed elsewhere than on aircanada.com (with either a travel agent or on another website)" followed immediately by the correct link. The normal "change seats" link below that is grayed out because that's for tickets booked directly with AC.

Either way, this is apparently much faster and easier than calling them on the phone with their notorious wait-times.