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.