There seems to be a new(ish) feature where you can change the customer on an order. This is done with the `orderCustomerRemove` and `orderCustomerSet` mutation. But these only seem to be available to the internal API as of now.
It also doesn't seem to be part of the release candidate and the unstable API version.
So this is me pleading–feature requesting–to have these mutations added to the Admin GraphQL API.