If you want to convert your classic SharePoint sites into modern SharePoint sites, there were released a new commandlets to PnP PowerShell which allows you to achieve this. 

ConvertTo-PnPClientSidePage

This is our current classic SharePoint Site:

Connect-PnPOnline -Url "https://tenantname.sharepoint.com/sites/sitename"

ConvertTo-PnPClientSidePage -Identity Home.aspx

This commandlet requires PnP PowerShell version 3.4.1812.0 (December 2018 release) or higher.

After sign in, the home site will be migrated to modern design. Instead of overwriting the home site it will create a new page “migrated_home.aspx”.

As you can see, there are some webparts that are currently not available in modern SharePoint. You will see a notification:

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