Orders can not be edited in Commerce. Makes sense.
What we want to do however is make it very easy for someone to re-order a previous order. So for example they've had an order that contains "two pairs of socks, two pairs of pants, toothpaste, and soap" but they need all of that again as a new order.
Rather than manually having to do so, we want to offer a "re-order" button.
Given that an order is just a cart that's been marked complete, we're thinking it should just clone the order into a new cart, so they can review it and make any amends they need to, then just continue as normal with the checkout process.
I can not figure out how to do this though. Getting the Order is simple enough, and there's an existing cart set up too. But how would I punt that old Order into the new cart?