Hi, mlewis...didn't notice this post at first...I've been paying a lot of attention to Xcel's mileage attempts this week... 60+ mpg will tend to do that...
Anyway, I don't plan on replacing the head unit, especially as I have the Navi, but I have already replace front / rear speakers. The ANC comes over the stock speaker wire, so if you leave it in place, ANC will still work.
That being said, if you put a power amp in, and rewire to the speakers, you'll lose the link to the ANC, which is in the head unit. I've read of another HAH owner on another site that told about this - it produces a g*d-awful noise for a few seconds, until it figures out that it's not working properly, and shuts itself off. According to him, it doesn't affect the noise level of the car at all when it's in VCM ('eco' mode), and that's the reason the ANC was supposedly added to the car.