If you replace a demigod file and want to revert to the old one, impulse makes this uber simple. Here's what you do:
1. Delete the file.
2. Right click on the game in impulse.
3. Click Verify, or Verify Download (not at my home comp, at gf's mac)
4. Wait. Impulse sees that a file is missing after it scans the game folder.
5. Click update. Impulse downloads the original file and you are set. (Again 4 and 5 may happen automatically, can not confirm this very second.)
Hope that is helpful for you and others. Should be very helpful for the modders.