It is an IATA rule that trumps the OW rules. It is not in their TOC because it does not have to be. First flight change means reissue against the conditions at that point in time. It is the law of the air and not a decision made by OW or your carrier. They can not make an exception. Something you could have taken advantage of if you purchased the ticket just before the 10% discount period. In this case: tough.

And involuntary downgrades are also no excuse since the ticket is for travel and not for a specific class. But that has been discussed to death already. Sometimes CX will compensate though, but only if it is last minute.