According to http://msdn2.microsoft.com/en-us/library/ms252109(..., converting RDLC to RDL should be very easy. In most common scenarios, there is nothing I need to do!
However, I found that VS (or SQL Server Business Intelligence Development Studio) will pop up this message box, saying "You have specified integrated security or credentials in the connection string for the data source, but the data source is configured to use a different credential type. To use the values in the connection string, you must configure the unattended report processing account for the report server"
Even after configuring unattended report processing account in reporting service configuration tool, it still keeps popping up. Finally, I found the difference between RDLC and RDL file generated by VS.
RDLC: <ConnectString>Data Source=.;Initial Catalog=JobReport;Integrated Security=True</ConnectString>
RDL: <IntegratedSecurity>true</IntegratedSecurity><ConnectString>Data Source=.;Initial Catalog=JobReport</ConnectString>
After I convert the upper line to the lower line, it works! Isn't that weird?