I believe this could be done. I have read this somewhere. Which Canadian store are to close to? If you are exchanging at the Canadian flagship in Toronto, they could be more accommodating. I find that the smaller Tiffany boutiques are not too accommodating.
The boutique on Sherbrooke is pretty big. Maybe you can call ahead to ask and to ensure that they have the smaller size.
Thank you! I called and received conflicting answers, ended up making the extra drive out to the original boutique. Too bad Tiffanys is marked up in Canada! Would solve all problems lol.