Change the product status after checkout_complete
You'll want to use the commerce hook_commerce_checkout_complete which runs after a order is complete (AKA paid for)

http://www.drupalcontrib.org/api/drupal/contributions%21commerce%21modules%21checkout%21commerce_checkout.api.php/function/hook_commerce_checkout_complete/7

That provides the order and you'll just need to get the product ID from that $order variable, load it with product_load(), change the value you want and then save