Any calculations associated with the rebooking will be made in the currency of the original country of departure (Point of Commencement). In cases where the ticket was issued in a different currency, it may be that different currencies were used in the calculation (ticket currency for any refunds, currency of departure country for any charges).
Unfortunately, it is not possible to rebook online in these cases. You will receive a message informing you of this with the request to contact your booking centre for rebooking.