ANSWER:

If I understood your question correct, I guess you can use the “-u” option.

   -u, --update
          This  forces rsync to skip any files which exist on the destina-
          tion and have a modified time that  is  newer  than  the  source
          file.   (If an existing destination file has a modification time
          equal to the source file’s, it will be updated if the sizes  are
          different.)

by Chakri from http://serverfault.com/questions/384003