Usually people that mod want access to older versions as time goes on. Impulse always automatically assumes to give you the most current version however. What you can do is archive your installation so that you may roll back to a previous version if ever needed.
To do this, click the Impulse orb in the upper left and then click 'Preferences'. Click the 'Archive & Install' listing on the left. You can toggle Impulse to automatically archive your install before downloading new updates. If you wish to do it manually, you can right click on the game from the 'My Games' tab and choose 'Archive Application'.