Overriding Add to Cart Button with Preorder On Some Products

I have a client who wants to be able to have customers pre-order items that are out of stock. I can see how to set the display to the sold out image, and replace the sold out image with a Preorder image, but then it does not have the button functionality.

I can also have them still be allowed to order those products, but then I still have the button_in_cart rather.gif rather than the button I want. Is there a way to change the button that is displayed for images that are out of stock, but still allow them to function as a normal item?

I assume there is somewhere in the PHP code where I'd have to change this?

or allow check out even with in-sufficient stock
Admin >Configuration> stock:
look for Allow Checkout
set it to true

Yes, that will allow them to checkout... But I can't keep the preorder image that I inserted into the Sold Out image filename.

I need the Add to Cart button to be replaced by a different image, but still allow them to check out. Any ideas?