The "Set as master" option should be available when in the Develop tab, not just when in the Library tab. I'm in the Develop tab when I'm making my edits and deciding which one I prefer. I shouldn't have to switch tabs just to change which image is the master copy. It would also be helpful for Set As Master to be an option when I right click on the image strip at the bottom (where I can already select to make a virtual copy), rather than having to go to the top menu bar.