Actually, you don't have to upgrade the reports themselves - they should work with no problems. However, as Jayashree indicated, you'll have to upgrade to the latest version of the SDK, which can be downloaded from here: SAP Crystal Reports, developer version for Microsoft Visual Studio: Updates & Runtime Downloads. Download and install the "Install Executable" for integration into Visual Studio. The other downloads are for the various types of runtime installs.
Here are a couple of things that Jayashree doesn't mention:
1. The version 13.x .dlls do NOT play well with the .dlls that come with CR 2008. So, you'll need to either uninstall the old SDK prior to installing the new one or use VS 2010 and the new SDK on a VM.
2. The Crystal SDK does not always work well when targeting your application to "Any CPU". So, you'll need to set the project properties to target either x86 or x64 and then install the correct version of the SDK runtime when you deploy it.
3. You MUST target the full version of the .NET Framework 3.5 or higher - the SDK does not work with the "Client" versions of the framework.
-Dell