While you could replace the heavy cream with whole milk to lighten the calories and fat, you also risk the integrity of the sauce and it won't be as thick or creamy. A secret trick I sometimes use to make the sauce lighter but still thick and creamy is to start with a roux of butter and flour and then use nonfat milk and Parmesan cheese. Check out my Chicken Alfredo Baked Ziti where I talk about this in detail.