Each order belongs to a customer. You can change the customer of an order when the order is still in its draft state.
An order remains in the draft state until:
- Payment is received
- Invoice is sent
- Order is marked as paid manually
Once an order is no longer in the draft state, the customer can no longer be changed or added. If you want to change the email address associated with an order, you still can regardless of the state of the order (Changing email address).