SQL Server 2005 Standard Edition can be upgraded to SQL 2005 Enterprise edition. Please refer support upgraded paths at http://technet.microsoft.com/en-us/library/ms143393(SQL.90).aspx
Follow the instructions mentioned in the MSDN article http://msdn.microsoft.com/en-us/library/ms144259(SQL.90).aspx#skuupgrade to upgrade SQL 2005 Standard Edition to SQL 2005 Enterprise Edition.
To put it simply, Launch SQL 2005 Enterprise edition setup.exe from command prompt using setup.exe SKUUPGRADE=1
For additional information and screenshots, Refer Page 61 from SQL 2005 Failover Cluster whitepaper http://www.microsoft.com/downloads/details.aspx?FamilyID=818234dc-a17b-4f09-b282-c6830fead499&displaylang=en Though this whitepaper is about SQL 2000 to SQL 2005 upgrade, steps almost remain the same
Edition upgrade must be launched from active node and once upgrade is completed, please apply SP3 from active node.
