Are you using the assignments editor? What are you changing? The .wid is read in by Designer on process load to set data inputs/outputs and icon for your service nodes; the wid itself has no bearing after that as far as how you manipulate your BPMN2.
If you are asking about how to change your workitem definition confuguration in Guvnor, you can click on your package, select the Assets tab, ho under Workitem Definitions and open it. It will be openend in the Guvnor Workitem Definitions editor. From there you can make changes and save them. Change will be picked up after you re-open your processes contained in that same package.