How do I Edit a completed order and Share cart..?

[Coultsy]

We have a client who takes orders with many line items.
Sometimes items are out of stock ad they need to edit the order and add replacement line items.
This can result in a new amount owing.
This process does appear to change the payment status to 'partially paid' which is great, but there seems to be no way to share a link with the user to pay the balance owing.

The 'Share cart' option is not available, I assume because this is not allowed when an order is 'completed'.

I can't see any way to re-open an order or remove the 'completed' status.

I was going to suggest that my client just creates a 'New order' but this is a giant hassle because apparently addresses are not stored against the user so the 'New Order' tool results in no address information.

Does anyone know of a workflow I could suggest to my client?