A user that is currently assigned a Chatter Free or Chatter Only license may be converted to a standard Salesforce license at any time. To do so, the system administrator simply needs to Edit the affected user's User Detail and choose the correct license from the User License picklist.
Here are the steps to follow to updating the user license from Chatter to Salesforce:
1. Go to Setup | Manage Users | Users.
2. Click Edit next to the user’s name.
3. Change the User License to Salesforce
4. Set the user profile on the Profile dropdown
5. Click Save
If a user is currently assigned a full Salesforce license, or a Partner license, this cannot be converted to chatter free of chatter external license type. The workaround for this issue would be to deactivate the current user and recreate that user with the new license type needed.
NOTE: When deactivating a user with this intent, be sure to append the username of the deactivated account with a ".old" (or similar) to prevent a username duplication error upon recreating the user.
Additional considerations for deactivating users may be found in the following Article: Deactivating Users
More information on License Types may be found in the following Article: Understanding User License Types