Nonstop to one-stop necessarily means that the nonstop is cancelled or you have otherwise been involuntarily offloaded (the latter very unlikely). In that instance, you are entitled to a refund even if the new itinerary gets you to your final ticketed destination at the exact same time. The contract provision as well as US law provides for a refund if your flight is cancelled or there is a significant change. You satisfy at least one condition, thus simply do not accept the change and request a refund. Follow up everything in writing (webform / email is fine) so that you have a paper trail if there is a hassle.