With the Email Designer, you can now build reusable templates and fragments from your email HTML created in the Legacy Editor. We have added a use case to allow you to convert legacy editor emails into the Email Designer. This use case enables you to create an Email Designer template by using an HTML email and dividing it into HTML components in the Email Designer.
To read the detailed documentation click here: Converting Legacy editor email content
For questions and feedback, please reach out through the below comment section.