You can find detailed information on how to upgrade RadControls for ASP.NET AJAX in the following online resources:
Upgrade the Version of Your Telerik ASP.NET Controls in 6 Easy Steps
- a simple, yet safe tutorial
Getting tired of seeing trial messages in your ASP.NET AJAX application?
- on determining the version of the Telerik.Web.UI assemblies your application uses
Upgrading RadControls Trial to RadControls Developer license or newer version
- a detailed list with upgrading instructions
Even if you are strictly following the recommendations in them, there still might be issues if there are custom configurations on your application (like custom styles, scripts, etc.). We have introduced many new features for the RadControls (and improved others) in the last two years, and this could be the reason for the experienced difficulties. I would suggest that you review the release history of the controls used inside your application and see whether there is any breaking change, leading to such a behavior: http://www.telerik.com/products/aspnet-ajax/whats-new.aspx#ReleaseHistory
If you describe in more details what are the exact issues you are facing, we will give our best to provide you with more to the point answer(s) on how to fix them.
the Telerik team
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