Ah, I see -- meaning in mid-game I'd need to shut down the program and rename, say, units.gif to units-old.gif, then rename units-new.gif to units.gif? (and the corresponding Rules file, obviously) Do I have this concept right?
Jeez, it's so simple, but I never thought of it. So you could effectively double the number of units?! But how could you make the "new techs" for them? Because if, say, Alphabet becomes Hypersonic Futurejobbery in rules-new.txt, then won't that cause problems in the game b/c Alphabet's not there anymore?