I just read on the Flying Blue Webpage that Air France is now offering "last minute upgrades" which are given out at the airport or even on board:

"If you decide to get an upgrade for your AIR FRANCE or KLM flight at the last moment, you can arrange this in person at the airport on the day of departure. It is also possible to arrange your upgrade on board your flight. You can do this for KLM flights from and to Amsterdam and selected AIR FRANCE flights. Please check the possibilities with the purser on duty. Last-minute upgrades, if available, can be booked only for the first segment of the flight."

Not sure if this has existed for a long time or whether this has been introduced recently.

Has anyone experience with this and knows what the rates are compared to a standard upgrade?