Roll-backs may work fine for certain packages in isolation, but I wouldn't ever expect a large-scale roll-back of an entire distribution to leave you with anything other than a horribly broken system (or one which, on the surface looks to be fine, but strange things keep surfacing as you discover it is more and more broken).
APT is just designed to avoid roll-backs as much as possible, and the packages that make up the Ubuntu (and Debian, for that matter) distribution aren't designed with smooth roll-backs as a design goal.
Unless you have whole-system backups that are reliable, what you really need to do is ensure your entire home directory is backed up (and possibly also on a separate partition), re-install from the CD image of your chosen version, and start restoring your data from your home folder(s). You may even need to selectively restore the dotfiles in your home directory in case any which have become accustomed to the new version don't quite work well with the older version.
Before you do, you may want to peruse your installed packages and write down which ones you rely on, so you can re-install them. There are tools to backup and restore your list of installed packages, but I wouldn't normally recommend doing this if you are going between versions as the packages needed/included may have changed.