Little Muffinn : The wooden brown

Tiny Muffinn : The spirit of elements
You can enable pre-order while creating or editing a product. This makes the Pre-Order button appear automatically when the product or one of its variants is out of stock.
 
For example, you can create a new product with 50 items in stock and enable pre-order. As long as the product is in stock, customers can purchase it. When the product is out of stock, the "Add to Cart" button changes to "Pre-Order".