but also in a way to tell the FA that you are second class J passenger, you're using a cert.
No one complains more about AC than I do, but I
always upgrade using a certificate, and my on board treatment is never less than sterling. I invariably get asked my meal choice first, regardless of where I'm sitting. And frequently I note subtle differences in my treatment, that is better than most other passengers in the J cabin -- a result of being a Super Elite I presume.
I don't think FAs give upgrades a second thought other than to think the upgradee is a very frequent (aka "high revenue") customer of Air Canada.