SQL Server 2012 Evaluation Edition was installed on a server as a temporary measure with a view of upgrading it to Enterprise Edition at some stage in the future. Thankfully the steps were quite straight-forward and did not require building a new server and migrating data etc. I had no idea that SQL Server 2012 was still on the Evaluation Edition until one day the backups started failing, no doubt after the 180 evaluation period had expired. When opening SQL Server Management Studio I saw the following warning:

The steps below will show you how to upgrade SQL Server 2012 Evaluation Edition to Enterprise Edition (same steps apply for Standard Edition).

The Upgrade Process

1. Locate the SQL Server 2012 install media and mount it to the desired server.

2. Run the media, select Maintenance and click on Edition Upgrade:

3. On the Setup Support Rules section, click OK:

4. Then click Next:

5. Enter your product key then click Next :

6. Check I accept the license terms and click Next:

7. Select the instance that needs to be upgraded, in my case it was SANDBOX, then click Next:

8. Assuming no failures are detected during the compatibility check, click Next to proceed:

9. Click Upgrade for the upgrade process to begin:

10. Once the upgrade process has completed click on Close:

11. To verify SQL Server 2012 has been upgraded successfully, login to the instance via SQL Server Management Studio:

12. Within Server Properties you will observe the Product is now listed as Microsoft SQL Server Enterprise (64-bit):

That’s it!

References:
Upgrade to a Different Edition of SQL Server (Setup)