Air France Frequence Plus - Possibility to upgrade to Premium Economy at Check In?

I'm flying yyz-cdg in August on a Delta issued ticket but Air France aircraft. A very discounted economy fare was booked, but we think we'd like to pay to upgrade to Premium Economy. We've called Delta and Air France and we're not allowed to make any changes to the ticket. Is it possible to upgrade and pay at the airport?

See:

http://www.flyertalk.com/forum/air-france-frequence-plus/1300082-air-france-rules-paid-upgrades.html

http://www.flyertalk.com/forum/air-france-frequence-plus/1089225-paid-upgrade-premium-voyageur.html