How do you convert an existing order into a shopping cart order?
As you mentioned a shopping cart is nothing more than an order with the state of cart.

So all you need to do is to update the state of $order to cart using commerce_order_status_update() function:

&lt;?php
$log = t('Reverting back to Cart state so the customer can modify items.');
commerce_order_status_update($order, 'cart', $skip_save = FALSE, $revision = TRUE, $log);


Alternatively if you are using rules, you can use the rule: "Set order status : Cart" which comes with commerce module out of the box.