CO booked you on AF in economy for the two segments in question. Any change will need to be made by CO. If you arrive at the gate and ask AF to upgrade you based on what you paid CO (in miles) you will be denied because of this. If you were flying on CO metal you could waitlist for the business upgrade and then claim the empty seats at the gate (see "Plan B for BusinessFirst in the forum FAQ). In this case you can not do this as you are on an AF flight. I suggest you periodically check with CO for availability.