Re: Adding a "coming soon" button to product display
You're more than halfway on track - you are on the right track for one way of addressing your problem.
Make that new button, save it as includes/templates/your_template/buttons/english/button_sold_out.gif
(note that there is also a small sold_out button (button_sold_out_sm.gif) - you might need to replace that as well.
There is another way to approach this: you could create a new product type and call it products_upcoming. Everything about that product type would be identical to the product_general type except that you set it as being unable to add to cart. It's a lot more work that simply changing the sold out button, and as you're selling all digital products you probably don't need the extra flexibility this approach provides.
An assumption is what you arrive at when you get tired of thinking...