Hit File -> Duplicate ... save the new copy as a different file name, in the original File -> Revert -> Last Opened. Now your Duplicate contains all of the new

Hit File -> Duplicate ... save the new copy as a different file name, in the original File -> Revert -> Last Opened. Now your Duplicate contains all of the new stuff, and your old file contains all of the old stuff. Duplicate and Revert Original should be an option, unfortunately it isn't.