Did LON-YYZ on a fortnightly basis for a year about 20 years ago but mostly on AC - back in the days when AC was in bankruptcy and desperate for business post 9-11 and also the SARS outbreak there. AC had a wonderful paper upgrade certificate scheme (SWU and SSWU) that were not linked to a flyers account and via Flyertalk, extra upgrade certs were freely available. Many seemed to be earned by AC regular fliers whose employer always flew them in J so the upgrade certs were worthless to them and they would just donate them to others.

I never paid for J (or rather my employer didn't back then) but via paper upgrade certs was able to take around 30 returns in J for the price of discounted Y - so each about £350 return back then.

AC has long switched to personalised electronic upgrade certs that are only redeemable by the flyer that earned them. They were good days though, times like that of endless J seats for the for circa £350 will never return