Once a settlement has reached "huge city" status, there's no going back and no possible way to change the culture. That's why it's a good thing to have a few key cities in the game, like Athens for example, to start out as huge cities from the very beginning of the campaign (see the file called "descr_strat.txt" in the "imperial_campaign" folder within "data"). That way some stupid barbarian faction can't just waltz in, capture the settlement, and make something that was always historically Greek into a barbarian settlement FOREVER.
