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.