Make them "settle" wherever they end. IF they end their turn in a non forested square, make a new forest there. if they end it in a new forest, upgrade it to a normal forest. If they end in normal forest, upgrade it to an ancient forest. (no benefit for ending in an ancient forest, you probably won't have any anyways). This would negate the annoyance of losing your AFs, and even enable you to effectively move ancient forests around, gathering them from farther away to bring them to cities that need them more.