If your question is on the day of departure, it exists from years, both with AF and KL. I've got offers when selecting a seat at the machines, sometimes during check-in. Cost really varies a lot: I've got an offer for 320 EUR from AMS to YUL with KL. I immediately accepted even if it was on the A330, which I would never pay the full fare for. Those upgrades, however, are really for the on-board seat and service, so you're going to get the miles of your original reservation.

I've no idea about the upgrade on board, I've never tried that. On AZ, that was very expensive.