There is a function zen_get_buy_now_button ... that controls the button setting for add to cart so that it changes based on store status, customer authorization, product status such as call for price, sold out etc.
Adding a new case to it that changes the button to a delete or something other than an add to cart vs the other choices was the basic idea ...
This saved having to touch the code in numerous places ... and just using the one function to control it all ...
In this circumstance, the idea was if a product existed in quantity of 1 or greater it would switch to a delete button ...
Adding this code before the default in the last set of case statements should manage this:
// switch to delete button when at least 1 is in the cart
case ($_SESSION['cart']->get_quantity($product_id) > 0):
$return_button = '<a href="' . zen_href_link($_GET['main_page'], m zen_get_all_get_params(array('action')) . 'action=remove_product&product_id=' . $product_id) . '">' . zen_image_button(BUTTON_IMAGE_DELETE, BUTTON_IMAGE_DELETE_ALT) . '</a>';