If you have extracted the Silverlight ReportViewer template
previously, you will have to extract the template again using the new assemblies versions in Expression Blend and take the new xaml.
In case the above does not help, let us know which versions of Telerik Reporting and Telerik RadControls for Silverlight are used, and what is the xaml of the user control containing the ReportViewer.
All the best,
the Telerik team
Have you tried the new visualization options in Telerik Reporting Q1 2013? You can get them from your account.