During our first tests with Salesforce Communities we figured out, that users can't be switched from Partner Community License (PCL) to a Customer Community License (CCL).
This is also documented here by Salesforce.com:
Now, that Salesforce.com introduced the Customer Community Plus License (CCPL), we would like to know if switching back-and-forth between CCPL and CCL on a user record is possible. If yes, this would enable us to have a mix of both licenses inside the system, and assign it to users based on demand for functionality.
Best Answer
The above does not seem to be the case nor as simple as implied. When moving from a customer community user license to a customer community plus user it is not as simple as changing the user license and profile on the user object. It would appear that you have to migrate the users ( create the new user as a customer community plus license, then deactivate the old one ).