I do not think this is possible - V8 Beta 1 will install over (replace) V7.
You can install on a second computer to try (see the notes about testing V8 install over V7 then reverting back to V7 below).
As a test I installed V7 Final on a second computer and checked operation, then installed V8B1 overtop and checked ops, then reinstalled V7 Final and everything worked ok on the reinstalled v7 Final.
Cannot guarantee, but reinstalling V7 over V8B1 to go back old version worked fine for me.
Recommend you backup the Newsleecher data folder normally located in C:\Users\*YOUR USERNAME*\AppData\Roaming (Note: you may have to go to Folder Options and select "Show hidden files, folders, or drives" to see this folder.)
To change folder options:
Roaming folder and how to get to folder options:
the Appdata\Roaming "Newsleecher" folder to something like "Newleecher.Original" or whatever different name.
Note: NL cannot be running when you copy/rename.
That way if you have a problem when/if you go back to V7, you can delete the "Newsleecher" folder and rename the "Newleecher.Original" folder back to "Newsleecher" Note: Anything you did since the backup will not have occurred - also after the backup make sure the files copied properly - both folder contents should be identical
By copying the C:\Users\*YOUR USERNAME*\AppData\Roaming\Newsleecher" folder, you are backing up Newsleecher's data files - useful for moving Newsleecher to another computer or to backup NL data - Copying this file to the new computer (Roaming folder) all your previous settings will be used by NL. Note: NL cannot be running when you copy/rename.
Copying to Roaming folder before installing NL and NL will immediately use your original settings (registration info, Groups, Folders and everything else) upon install.
If you have changed the Newsleecher data file location by placing a "newsleecher.ini" file in your NL program folder then copy and backup that folder.
Sample Newsleecher.ini file:
My Newsleecher datafile location (since NL version 4 - Allows easy backup of NL data):