I run Quickbooks Desktop for Windows on my Mac via VMWare Fusion so I was able to simply restore the VMWare file that immediately preceded the R13 update. All of my memorized reports now appear as they were before the update.

For those running QB Desktop directly on a Windows Machine I know of no way to roll back an update, however If Quickbooks pulls the R13 update it will allow you to uninstall QB then reinstall and run the updates. If Quickbooks does not pull the R13 update than more than likely a reinstallation will be too far behind and will not open for Quickbooks file without forcing you to update. Perhaps someone can do some testing to help out the community.