After installing SP1 on SCVMM 2008 it converted to trail version.
1. Download the correct licensed version of VMM 2008 R2 SP1 from the above link.
2. Uninstall VMM 2008 R2 SP1 evaluation with retain database option
3. Start the install of the downloaded VMM 2008 R2 SP1 Licensed version and select the retained database.
Note: During reinstall you might get below error, when you click the drop down for the SQL Instance:
"No Sql Server instance is found on the specified server localhost. (ID:360) verify that SQL 2005 is installed properly and the SLQ Server service is running"
To solve this issue uninstall the evaluation Admin Console and try to install again.