While working on telerik control upgrade projects for asp.net weI found different website, issue that consume a lot of time infact more than what I was expecting for this activity.
1. Check the version you are using as old telerik controls were available with multiple dlls i.e. separate dll for each control.
2. Check namespace of your controls. As old were using telerik.wecontrol while new one use telerik.web.ui
The main issue that we face during migration was that we were using different version of DLLs previously and thought that if we use new version it will work. We we started using new dll we realize that AJAX was not working and shifting of all code to new code will be required. I have found different issues which I will describe separately in different blogs.