Found this information in the IBM Lotus Knowledgebase ( Technote #1438255 )

The working sets information is kept in a file called workingsets.xml in the following location in your Notes installation folder:

Notes\Data\workspace\.metadata\.plugins\org.eclipse.ui.workbench

Copy this file to the new Domino Designer client in the same location while Domino Designer is not running. When you next start up Domino Designer the working sets will be available and matching the previous client.