Havent done this in few years, but last time it was easy to have both old and new version of a game through "appmanifest" files. You just need to make steam think it has all the updates in there (best would be to have a manifest file from when the patch you are trying to run was "live" as guideline).

The only real limit was that to change patch you needed to restart steam and move/rename one of the manifest files (without one steam wont see the game files).

For bigger games its even better than steams own beta tool as it wont download anything every time.