Charge for Flight Date Change

Question

Im having trouble understanding the rules. Does that mean changing the return flight will be Eur 90 or free? Cause 4. 1/ states free of charge... Im assuming in case of class change, Ill simply pay the difference or is it the difference + 90?

Flight is in Asiana OZ 542 and 541 in return.

Also is this kind of thing always the same for the same airline or does it depend on the ticket? Because if it is the same, then I have changed the date on that same flight before for free but I wasnt sure since this time it's a cheap ticket from a travel agent.

Changes

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.

Answer 1

The fare rules you've listed allow you to change the date of the return flight only (NOT the outbound flight which is the "1st Flight Coupon") for free, but only if the new flight you are changing to meets certain conditions (same "Routing/Carrier/fare Amount/fare basis/stopover Condition")

If you are simple looking to change to the same flight on a different day, then the most limiting of these will likely be the "fare basis" and "fare Amount". Basically this means that if your original ticket for that leg costs €100, then there has to be a €100 fare available for the new date in order for the change to be free. As airline fares frequently change over time, it's very possible that the €100 fares will no longer be available, and the new price will be €150 - at which point you will need to pay both the additional €50 fare, PLUS the €90 change/reissue fee.

(The "fare basis" restriction actually means it's possible you'd be charged the fee even if the actual flight price was the same but the availability in the fare bucket you booked was sold out - but that's unlikely, and rather confusing, be best to just ignore it)

The exact fare conditions, and related fees, will vary depending on the airline and the specific fare you purchased. eg, for a specific airline, a €80 ticket might not allow any changes at all, a €100 ticket might have the conditions above, and a €200 ticket might allow free changes.