In order to change order customer or assign guest order to a customer, you need Magento 2 Convert Guest to Customer extension since it enables you to do it just in one click.
This extension allows you to automatically convert guest orders to customers based on email address. However, sometimes customers place guest orders with different emails and want these orders to be stored in one account.
That is exactly when "change order customers" feature comes into play.
To assign order to customer in Magento 2:
1. Navigate to Sales > Orders and choose the order you want to be assigned to another customer.
2. Then you simply press the Change Customer button in the Account Information section.
3. Choose a customer you want to assign to the order from the appeared grid of customers and press the Assign to Order.
After that order customer will be successfully changed and assigned to the right customer account.
Magento 2 Guest to Customer allows you to provide better support and experience for your customers. It also makes customer management way easier.
However, the "change order customer" feature is not the only option this module offers you.
You can also subscribe converted customers to a newsletter, add them to the corresponding customer group and even convert all old guest orders to customers in once click.
Have you already explored all of these features?