For full-fare tickets you will probably need to call to book the tickets as the website does not allow to do this. AF's website alles a "full flex" option (depending on destination) which removes change fees but does involve fare readjustments. If you book the fullest of flex fares (i.e., full fare M/Y or C/J) then there will not be any fare readjustment when changing flights are you are already paying the maximum fare.