Award tickets have used married segment logic for several years now. It's no different than with a revenue ticket. If you bought a revenue fare on these flights where the fare had a 3-week advance purchase and wanted to make this change within 3 weeks of your flights, you would no longer qualify for the 3 week advance fare and would be expected to pay the fare difference. Or if the new flights simply no longer had the same fare bucket inventory available as the flights you originally booked, again you would have to pay the fare difference.