You don't really need a copy of a separate version.
You can install the beta patch any time by unpacking the rar file in the main game folder.
Then, if at any point you wish to go back to the latest official patch just open your Steam library, right click on the CoD icon and select "properties". In there, navigate to the "local files" tab and click on "verify integrity of local cache".
This will compare all game files with the official patch and if anything is different, it will download it again and replace it. Essentially, it gets you back to the latest official version, removing the beta patch from your installation.
