REST API Weirdings

I tried the REST API to update an order according to the documentation but got the following error:

{"customer_id":{"required":"Customer selection is required"},"order_items":{"required":"Item selection is required"}}

Why is there a customer selection required if I just want to change the order (not the customer!). Also, order_items is an array. Do I really need to enter all items of that order to just change the whole thing?

This seems not to be working or the documentation is far off. Is there a working example somewhere?