You can't keep backups of the patch files because there aren't any - Impulse just does a file compare and downloads/overwrites the changed files.

What you can do is archive the entire installation of the version you want to keep, and when you install from it you'll get that version back.

To make an archive just right click on the game in Impulse and you'll see the option. Default archive path and the ability to change it (as well as automatically archiving the game before patching) are in the Impulse orb menu in the top left > Preferences > Folders.