[Baron Munchausen]

Yep, Mega-Evil can change. It's especially interesting in single player when an AI gets there first because you can ally with the other AIs and gang up on it, but your superior planning and organization skills will lead you to replace it and they will eventually all turn on you.

Also, by changing the Mega-Evil reaction in the AI_Politics files you can reduce the 'instant enemy' effect and make a much more realistic shift of attitude that takes several turns.