Thanks to Cos (everything started in
here), a new neat function is now in place.
In edit orders page there's now a "Is registered user?" yes/no label on top of user details, along with a "manage" button.
By clicking that button you will be able to assign/unassign the order to any registered user you have.
As wrote in the other thread, everything should work fine: but as usual, test and let me know in case of any issue!