Just asking, but why would you want to downgrade?  Sure, the units are different now than they were at first, but if you don't have the updates, you can't go Ironclad Online.

  Now, the only thing i could think of is copying the new Entrenchment file, putting it somewhere safe, uninstalling the game, then replacing the files where you find them.  It might not work, however, because it's possible that the latest patch is required for some of the newer units and abilities to function properly, or keep the Starbases balanced.  There's probably another way, i just don't know what it is.

  If you're just worried about being able to play local LAN games, then try copying the entire Sins Folder and replacing the one on your other computer.  Again, not sure it will work, but it's worth a try.  Be sure to keep the copy so you don't lose any saves/achievements you might want.

alternatively, you could always go to http://anywhere.impulsedriven.com/ and download the entire game for an offline installation including the latest patch.

you will need impulse installed on the target computer, but no internet connection is required. this is officially supported, so you can use your normal account on both machines.

the only thing you won´t be able to do is play online at the same time, but LAN should be no prob.