There are cases when you need to convert all your old guest orders and create customers on their basis. It can be easily done with the help of Magento 2 Convert Guest to Customer Module.
1. Once you have this module installed you need to navigate to Stores > Configuration > Magefan Extensions > Convert Guest to Customer.
2. Press the Convert All Old Guests to Customer button.
3. After that, you'll be asked to confirm the action and once you've done all guest orders will be converted into customers and you'll see the success message.