This seems to be an issue with the reference of the RadControls DLL files in your project. For example it may be possible that you have successfully updated the old DLLs in your BIN folder, but the project references some of the DLLs from the GAC, which are different version.
Please find attached a short video that demonstrates how to check the DLL references in your Visual Studio project.
What I can suggest you is to have a look at this
blog post, that gives you 6 easy steps on how to upgrade/revert your version of RadControls.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed