You may notice that in CRM 2015 Update 1, when you click to a different stage in the process flow, the “Next” button changes to “Set Active.” But you may also notice that it is grayed out and doesn’t do anything. The “Set Active” button is designed to allow users to jump back to an already completed stage. if you click on a stage in the past, you can click “Set Active” and move back to the completed stage. It cannot be used to move ahead to a different stage. Thanks to Chris Treptow for helping solve the mystery.