When you create a new D365BC demo database, you will always get the essential demo data. Even if you switch the experience from essential to premium, the data will still remain in essential.
To get the premium data, Microsoft already have a Predefined Packages for you to import.
Go to Configuration Packages and click on Process > Import Predefined Package
Select the Extended package and click OK.
Once imported, you just need to Apply Package and you will get all the premium sample data in it.