Check this out:
The 180-day trial version of SQL Server 2005 Enterprise Edition can be upgraded to the fully licensed version of SQL Server 2005 Enterprise Edition without uninstalling the trial software prior to the end of the 180-day period. If upgrading to the fully licensed version of SQL Server 2005 Standard Edition, Workgroup Edition, or Developer Edition, it is recommended that you uninstall the 180-day trial version of SQL Server 2005 Enterprise Edition before installing the fully licensed version of the other editions. Please refer to the product documentation for information about upgrading to the fully licensed software.
http://blogs.msdn.com/sanchan/archive/2006/07/27/681086.aspx