I often get asked about upgrading SQL Server 2008 Evaluation Edition to a retail edition such as Standard or Enterprise Edition. This is very well documented at the below SQL Server books online link:

How to: Upgrade to a Different Edition of SQL Server 2008 (Setup)
http://msdn.microsoft.com/en-us/library/cc707783.aspx

Below are the main steps, using the Setup , edition upgrade wizard.

  1. Insert the SQL Server installation media. From the root folder, double-click setup.exe or launch the SQL Server Installation Center from Configuration Tools. To install from a network share, locate the root folder on the share, and then double-click Setup.exe.
  2. To upgrade an existing instance of SQL Server 2008 to a different edition, from the SQL Server Installation Center click Maintenance, and then select Edition Upgrade.
  3. If Setup support files are required, SQL Server Setup installs them. If you are instructed to restart your computer, restart before you continue.
  4. The System Configuration Checker runs a discovery operation on your computer. To continue, click OK.
  5. On the Product Key page, select a radio button to indicate whether you are upgrading to a free edition of SQL Server, or whether you have a PID key for a production version of the product. For more information, see Editions and Components of SQL Server 2008 and Version and Edition Upgrades.
  6. On the License Terms page, read the license agreement, and then select the check box to accept the licensing terms and conditions. To continue, click Next. To end Setup, click Cancel.
  7. On the Select Instance page, specify the instance of SQL Server to upgrade.
  8. The Edition Upgrade Rules page validates your computer configuration before the edition upgrade operation begins.
  9. The Ready to Upgrade Edition page shows a tree view of installation options that were specified during Setup. To continue, click Upgrade.
  10. During the edition upgrade process, the services need to be restarted to pick up the new setting. After edition upgrade, the Complete page provides a link to the summary log file for the edition upgrade. To close the wizard, click Close.
  11. After installation, the Complete page provides a link to the summary log file for the installation and other important notes. To complete the SQL Server installation process, click Close.
  12. If you are instructed to restart the computer, do so now. It is important to read the message from the Installation Wizard when you are done with Setup. For information about Setup log files, see How to: View and Read SQL Server Setup Log Files.

You can also perform the upgrade using command line option, see below SQL BOL topic.

Command Line option

http://msdn.microsoft.com/en-us/library/ms144259.aspx#Upgrade

Hope this can help save time in upgrading to retail editions of SQL Server from the evaluation edition.

Note: The Evaluation edition of SQL Server is based on the Enterprise Edition and thus it is recommended to upgrade to SQL Server 2008 Enterprise Edition so you can continue to enjoy the many capabilities such as Resource Governor, Database compression, Backup compression, Transparent Database encryption, etc.