If you wish to change your seat (without changing the comfort class), a service on board (PSP, DVD player, etc.) or a supplement (animal or luggage), the exchange will not generate extra costs.
However, if you wish to change the day and/or time of your departure, your comfort class (first or second class) or your journey (place of departure and/or arrival), it will cost you €10 per passenger, per journey, as well as the possible price difference between the old ticket and the new ticket.
Example: if you have booked a Paris - Bordeaux return ticket for 3 people and you wish to change all your tickets, it will cost you 3 x 2 x 10 = €60 (if the new tickets are the same price).
iDTGV, Élu Service Client de l'Année 2012