Switch the “Add To Cart” button on Free Download products.

Hi, my shop is a digital download place and I’d like to switch the “Add To Cart” button with a different button in all the free download products with a “Download Now” button

So that different button will do something else.
I have a friend who currently works on what the button will do but we dont know how to only show that button in all products which are Free (Price 0$)

Any idea how to do this?