You can also find previous versions of your diagrams on the attachments page. Just click the arrow at the front of the listing to expand a list of all previously saved versions of the file.

Gliffy Images: Click on the version number to view the PNG file.

Gliffy Diagrams: Click on the version number to download the diagram as a Gliffy GON file (or a Gliffy GXML file, if you are using an older version of the Gliffy Confluence Plugin).