Thank you for replies
I did some hunting on AF website and came up with following which applies .
3.4. Flight Coupon Order of Use

(a) The Fare applied on the Ticket issue date is only valid for a Ticket used fully and in the sequential order of Flight Coupons, for the specified journey and on the specified dates. Any non-compliant use may result in the payment of an additional fare under the conditions defined below.

(b) The Gross Fare established on the basis of the details, flight dates and routes mentioned on the Ticket corresponds to a departure point and an arrival point, via any Stopover scheduled when the Ticket was purchased, and forms an integral part of the Contract of Carriage.

(c) A change in the departure or arrival point for the journey by the Passenger (for example, if the Passenger does not use the first Coupon or does not use all the Coupons or if the Coupons are not used in their issue order), may result in a change in the Gross Fare initially paid by the Passenger. Many Fares are only valid on the dates and for the flights specified on the Ticket. In the event of a change as outlined above, the Passenger may be required to pay an additional fare [or be entitled to a refund, as the case may be] equating to the difference between the Gross Fare initially paid and the Gross Fare that the Passenger should have paid when the Ticket was issued, for the journey actually made by the Passenger.
In the event of a change, Administration Fees will be applied, where applicable.
Again with thanks.