You can change a workspace's parent by selecting its icon in the Workspace Graph pane and then choosing Edit > Parent.This opens the Parent dialog box.
When the window is initially opened, the New Parent Workspace Directory text box contains the name of the current parent. Edit that line so that it contains the name of the new parent file and click the Parent button. The Workspace Graph pane is automatically adjusted to reflect the new relationship.
If you do not specify a parent workspace in the New Parent Workspace Directory text box, the workspace is orphaned--it has no parent. The Workspace Graph pane is automatically adjusted to reflect its new status.