Making a Feathered Fade

I'm working on this Flash project for one of our instructors, and he would like to incorporate a yin yang symbol in the animation. Is there some way I could incorporate some kind of feathered fade animation (starting at the tips of the black and white) to gradually swap the colors? It would almost be like a gradual fill effect starting from the tip going to each of the circle parts of the yin yang...if that makes any sense.

make yin yang parts into masks and slide gradients below in a tween.