Hello,
I have solved this issue. I have downloaded SQL Server 2008 Standard R2 from the Microsoft Licensing website. Clicked on the setup.exe, and navigated to Maintenance -> Upgrade Edition. In the series of steps it actually upgraded some of the files, and the license key is also embedded.
Before and after I have run "SELECT @@VERSION" command in SQL Management Studio, and the Evaluation Edition is now changed to Standard Edition.
This is called In-place upgrade and here we need not uninstall or install anything, it is pretty simple and takes less than 10 minutes.
Thanks,
Vineela.