In Salesforce, User License types are tied to Profiles, so to change the User License type of a User, you need to change their Profile. However, only certain types of transitions are allowed:
aka "Salesforce Licences". You can transition users between these six core licenses, but you can't transition from these six to any of the others. That is, you can't convert to Chatter or Community/Portal licenses.
- Salesforce Platform
- Force.com - App Subscription
- Force.com - Free
- Partner App Subscription
- Work.com Only
- Chatter Free
This license can be changed to any of the six Core Licences listed above - but then you cannot change back
- Chatter External
This license can be changed to Chatter Free or any of the six Core Licences listed above - but then you cannot change back
Customer Community Licenses:
You can transition users between these four Customer Community type licences, but you can't transition from these four to any of the others. That is, you can't convert to Core Licences or Chatter, etc.
- Customer Community
- Customer Community Login
- High Volume Customer Portal
- Authenticated Website
Partner Comminity Licenses:
You can transition users between these four Partner Community type licences, but you can't transition from these four to any of the others. That is, you can't convert to Core Licences or Chatter, etc
- Partner Community
- Partner Community Login
- Gold Partner
- Silver Partner
In summary: You can transition from Chatter to Core, but apart from that you cannot move between Core, Customer Community and Partner Community licenses.
To find out more details about the various User License types, see Salesforce Help - User Licenses Overview. But make sure you've got a decent hat on, because you're going to be scratching your head. A lot.
edit: Along similar lines, I just found this great blog post by Marty Chang - Allowed User License Changes: Upgrades, Downgrades, Switches which uses a large matrix to show all the allowable license transitions.