If you have access to the filesystem and are not afraid of modifiying it, then you could do this :
Install the
searchindex plugin.
Backup everything and make sure you known how to, and can actually restore. (that includes files ownership, permissions and/or other security properties of the files)
Then
1) go to the data folder and zip (or tar or rar or ...) just the
pages[/m] and the [m]media[/m] folders. Let's call that file [m]current-state.zip
2) then use the move plugin as suggested by turnermm, and move all your current pages and media to the "previous-version" namespace. Be sure that the move plugin moves media files too (I don't remember if it does this by default)
3) Once this is done, unzip
current-state.zip (adjust file permissions and ownership if needed), this will recreate your pages and media as they were before you moved them. You now have :
-) original pages and media (with their history) in the "previous-version" namespace
-) a new copy of these pages and media in the normal namespace.
BUT, those pages and media have no history and they are not indexed by DW
4) So, use the searchindex plugin to reindex your full wiki.