While it is technically possible to switch between Photo Mechanic 6 and Photo Mechanic Plus, we do not recommend it.All the features of Photo Mechanic 6 are already included in Photo Mechanic Plus. We recommend that you uninstall Photo Mechanic 6  after you upgrade and use Photo Mechanic Plus for everything from culling and tagging to filtering and catalogs. 

If you do use Photo Mechanic 6 to edit or move photos in Photo Mechanic Plus catalogs, your catalogs will get out of sync with the changes you made outside the program. Go to Catalog>Catalog Sync... to fix this.