Originally Posted by irishguy28

No. You cannot upgrade with miles during check-in.

Snagging a mileage upgrade is best done well in advance. AF/KL now have a model whereby every seat can be upgraded, for a sliding scale of miles, meaning that there are far more tickets eligible to upgrade with miles than used to be the case. This also means, however, that the chances are increased that the maximum number of seats they were willing to see upgraded using miles has already long ago been reached, and that other passengers have got in there ahead of you.

They are also more competent at selling upgrades for cash, meaning that there are far more avenues now for people to convert Economy to Business - so the chances of having a seat open for you to upgrade with miles at this late stage is slim. The time to try upgrading with miles is well in advance; conversely, the time to try upgrading with cash is at the last minute. (That the last-minute offer is so high suggests that they have a pretty full business cabin already, also bad news for your chances).

It is highly unlikely that there will be any seat available for mileage upgraders, but you can check when you get to the airport and see.

I would also recommend that you do not change FFP numbers in any booking. Changing the FFP number can lead to unintended consequences and miles ending up in the wrong account, even if, at the time of travel and boarding pass scan, the boarding pass appears to indicate the "correct" intended mileage programme.

You may not even need to have an FB number in the booking in order to upgrade with FB miles, so I would really recommend you do not put that number in the booking at all if your intention is to have the miles post to your Skymiles account without difficulty.