Cargo por cambio de fecha de vuelo

Tengo problemas para entender las reglas. ¿Significa eso que el cambio del vuelo de vuelta será de 90 euros o gratuito? Porque 4. 1/ dice que es gratis... ¿Supongo que en caso de cambio de clase, simplemente pagaré la diferencia o es la diferencia + 90?

El vuelo es en Asiana OZ 542 y 541 de vuelta.

Además, ¿este tipo de cosas es siempre igual para la misma aerolínea o depende del billete? Porque si es lo mismo, entonces he cambiado la fecha en ese mismo vuelo antes de forma gratuita, pero no estaba seguro ya que esta vez es un billete barato de una agencia de viajes.

Cambios

Any Time
  Changes Permitted For Revalidation.
  Charge Eur 90.00 For Reissue.
  Waived For Death Of Passenger Or Family Member.
     Note -
      1. Waived For Death Of The Passenger/immediate
         Family Member
         - Valid Death Certificate Is Required.
      2. Changes Only Permitted To Booking Class Of
         Equivalent Or Higher Hierarchy.
      3. When Changes Made To The 1st Flight Coupon Of
         The Unused Ticket In Terms Of Date/flight/class
         Of Service Or Sector
         - Ticket Must Be Reissued Based On The Iroe/
           Fare Level/fare Rules/fuel Surcharge/taxes/
           Exchange Rate Of The Ticket Reissuance Date.
         - Reissue Charge Must Be Collected.
         - For Fares With Ap Rule - Ap Rule Must Be
           Applied To Reissue.
      4. When Changes Made To Other Than
         The 1st Flight Coupon
          1/reissue For Revalidation To Change
            Date/flight Including Other Airlines
            Portion Is Permitted Free Of Charge.
            - Provided No Changes Are Made To Routing/
              Carrier/fare Amount/fare Basis/stopover
              Condition.
          2/name Spelling Change Is Permitted
            Free Of Charge.
      5. When A Reissue Is Required
         - New Ticket Should Be Issued As Below
          1/before Departure
            - Apply The Iroe/fare Amount/fare Rules/
              Fuel Surcharge/exchange Rate Of The
              New Ticket Reissuance Date.
          2/after Departure
            - Apply The Iroe/fare Amount/fare Rules/
              Fuel Surcharge/exchange Rate Of The
              Original Ticket Issuance Date.
      6. In Case Of Fare Combination
         - The Most Restrictive Rule Applies
           To The Entire Pricing Unit.
      7. Charge Applies Per Transaction And
         Per Pricing Unit.
      8. Child/infant Discount Does Not Apply.
         Infant Without A Seat Free Of Charge.
      9. Changes To Db Rail Sectors Permitted
         At No Charge.
      10. Refund/reissue Charges Are Non-refundable.
      11. In Case That The Transaction Made At Other
          Airlines Office Than Oz Offices Then The
          Service Charge May Be Collected From Passenger
          According To The Concerned Airlines Policy.