Updating WFScenario through a Business Rule

Each user's default workflow setting is stored in the WorkflowView.xml file in the File Explorer (see screen shot).  Unfortunately, there is no built-in call to update this for each user.  You would need to build a rule to loop over the XML file for all users and modify each. You would also need to be careful not to assign a scenario that a user does not have access to view.

Something similar to this is done in the AST solution where the default home page is updated in the UserAppSettings.xml file in the same folder.