You can't use miles to upgrade at check-in, this needs to be done with Flying Blue before the flight.

Now, I don't believe the FA at check-in has anything to do with your upgrade. Every time it happened to me, I only knew at boarding, as it was probably decided by the ground staff seniors whom you don't see but who check the filling of each class (and then decide to upgrade you based on your FFP level, the price you paid and also possibly your country of residence - my best bet is that non-France based PAX get better treatment).

Last, IMO you'd better fly when everybody else does (Friday to Monday, first and last days of national holidays), as you will have a better chance of being upgraded is the coach class is overbooked.

Stéphane