just to correct the OP, Delta does not offer SDC either for intl. itineraries. You can change the award ticket for free if it's more then 72 hours before the departure of your original flight. As long as O-class is available you can call Delta and have them change your AF flight to BOS; O-class is often coming up 3-5 days before departure. 2 weeks ago I changed a BOS-ATL-CDG to BOS-CDG 3 days before departure w/o any issues.