I think that is not true though, but others here could correct me. The contract between you and them is to be put on a flight between the cities you wanted on the day you wanted. There is nothing in the contract about the particular fare rules or how much it should cost to change the ticket, which is a completely voluntary decision on your part. Maybe out of trying to be consistent and courtesy to not surprise the customer, they are only applying it to future tickets, but I think they could legally charge (and change the requested charge at any time) to any amount to change the ticket if they wanted. Being able to change the ticket, the fees, and particular fare basis, are no part of the legal contract, I think, but as I say you can correct me, please.