You can use Control Panel / Licensing to convert from per server to per seat, but not from per seat to per server.
To convert from per server to per seat:
1. Use Control Panel to open the Licensing applet.
2. Check Per Device or Per User. You may receive:
You have requested a change that may violate the license agreement. The License for a one-time change from a Per Server to a Per Device or Per User configuration. Do you want to cancel this request?
3. Press OK to continue.
4. A windows displays:
Per Device or Per User Licensing
If you have not purchased a client access license for every computer that will access windows server, you must purchase them prior to using Windows Server.
For complete terms and conditions governing the use of this product, see license agreements, which can be found under help.
I agree that:
I have read and agreed to be bound by the license agreements for this product.
5. Select I agree and press OK.
NOTE: See the following Microsoft Knowledge Base articles: