This feature is available in Enterprise and Ultimate editions.
To reassign an item that is already assigned to a Backlog Group, simply drag the item to another Backlog Group.
Note that if an item in one project is assigned to a Backlog Group that sits in a lower level project, the item will be automatically moved down into the same project as the Backlog Group.
When the All Projects level is available and selected in the Project Navigator tree, Backlog Groups are not visible. Select a specific project to view and manage the Backlog Groups applicable to that project.