You can use coconut milk, although it will impart a slight coconutty flavor. If you are looking to decrease the calorie count with something like unsweetened almond milk, you could replace the whole milk with almond milk. You still need some fat for the recipe to turn out, so you will need to keep the heavy whipping cream. The only exception to this would be if you were using something like the Ninja Creami. I have recently bought the Ninja Creami Deluxe (Kohl's) (Amazon link) specifically to make ice cream with plant-based milks and protein drinks. The Ninja Creami is really awesome for this.