Workspace rebasing is the ability to change the stream on which a workspace is based on. Rebasing a workspacing is a lot quicker than creating a new workspace because it only needs to download the files which are different between the two streams. If the files between the two streams are the same then they will not be touched, so you will not need to rebuild all your files.
To rebase a workspace in the Workspace View simply right-click the workspace in the top left pane and select the 'Rebase Workspace' menu. Alternatively you can rebase a workspace within the Workspaces View using the 'Rebase' button. This will launch the Workspace Details dialog.
You can now select the new stream from which the workspace will be based. You can also modify the Display Name if necessary. When you are ready press 'OK'.