Are you kidding me? UA has the worst SDC policy if you read it carefully: "When the original ticketed fare class is not available on the requested flight within 24 hours of departure, the same-day flight change fee (instead of the change fee stated in the fare rules) will apply, as well as any additional payments or refunds for the difference in fare." So of course UA's Revenue Management is gonna make sure most flights don't have your original booking class available.