To the original OP:

The change fee depends on the fare conditions, but usually it is EUR 50 (if J class is available for your new flights - otherwise you will need to pay the fare difference to the higher booking class available).

Please remember to re-schedule your return flights/to do the rebooking before the return date of your original booking - otherwise you will be charged a EUR 100 no-show fee too. I learned this the hard way some time ago when I cancelled shortly before my return flight from BKK but made the new reservation for the return flights some days later (after the original return date) - no-show fee had to paid.

You need to pay at the local ticket office or at the airport before the return flights. I had no problems paying at the ticket counter at BKK until 2 hours before my flight.