When you click on the properties page of a user in the CMC, you get the following screen:
Notice there you can set the user as concurrent or named. You can do this for any user.
Assuming your license allows it, you can switch some named users into concurrent users.
If you want to see a summary of what your license key allows, you can click on the "License Keys" page in the CMC which will give you a summary of your named and concurrent user information.
If your license only has named users, then as Henry says, this is a license conversion conversation.