Imagine the situation when your customer contacts you and asks to combine all his orders into a single one. It may be challenging as by default Magento won't allow to do this.
Using the extension you can assign orders that are placed as guests ones to existing customers' accounts from the order view. Now you are able to reassign multiple orders in bulk to a customer as a mass-action. If a customer uses the same email while placing a guest's order, you can assign new orders automatically on checkout. After order reassignment customers will get the notification by email.
The extension a perfect tool that helps you reassign orders to customers' accounts.
Check the Reassign Order extension for Magento 2
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