When you have SQL server 2005 Enterprise in your PC, and you want to upgrade to SQL Server 2008 RC0, you will find the upgrade fails. That’s because the current SKU of SQL Server 2008 RC0 is Enterprise Evaluation, not Enterprise.
It is impossible for you to upgrade from SQL server 2005 Enterprise to SQL Server 2008 RC0 Enterprise Evaluation. We need to wait until SQL Server 2008 RTM Enterprise is released.
The only thing you can do is install previous SQL Server 2008 CTP first and then upgrade it to SQL Server 2008 RC0. The upgrade will pass.