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