Can I change the difficulty level of a save game in Alpha Centauri?

As far as I know, there is no way to edit difficulty of a saved game. You can only change difficulty at the start of a new game entirely.

You can change the difficulty level at any point in the game by activating the Scenario Editor. The only problem is that this will make you have a "Cheated" message displayed on your Alpha Centauri Score. On the main menu, go to Scenario -> Activate Scenario Editor. This will enable the menu option Scenario -> Set Difficulty Level. After you are done, once again click on Scenario -> Activate Scenario Editor to deactivate the Scenario Editor and resume regular gameplay.