How do you convert an existing order into a shopping cart order? I would like to be able to have an existing order that has already been checked out back into a user's shopping cart.  From what I have read from the documentation, a shopping cart is just an order for a user.

This is relevant because when my user's checkout the order, it only does an authorize only transaction, which means the funds have not been transferred yet.  I want to allow my user's a window of time to change their order.

So, I figured the best way to do this was to take the order that they want to edit, void it out, and then put it back into their shopping cart.

I can't seem to find an api call for this.   I saw hook_commerce_cart_order_is_cart_alter in the commerce_cart API, but I couldn't figure out when this was called or if this was a use case for this API call.
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.