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.
I'll make a few corrections. The Contract of Carriage provides that CO will attempt to transport you: if they can't do it in the air, the carrier could use another mode of transportation. If all else fails, the airline can cancel the ticket and refund your money.
The fare rules become part of the overall agreement between passenger and Continental once the ticket is purchased. Those rules don't change--unless there is a provision to allow them to change.
Here's the link to the Contract of Carriage.