Microsoft have placed on their roadmap that you can run the following PowerShell command:
to upgrade a classic site collection to a modern site collection.
However, the documentation at:
Enables the modern communication site experience on an existing site. At this time, based on early adopter feedback, we have decided to postpone rolling out this feature. We apologize for any inconvenience and will communicate a new schedule via Message Center, once it is available. We expect to have an update in the Q3 time frame
and when you actually try it you get:
So it looks like we’ll have to wait a little longer. Hopefully not too much longer.