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Changes and Backward Compatibility
A complete list of all changes can be found on the Release History page: https://www.telerik.com/support/whats-new/aspnet-ajax/release-history.
You can also use the Telerik API Analyser to make your upgrade process easier by comparing the version you upgrade from with the one you upgrade to.
The following sticky thread may be helpful with the upgrade too: Known Issues and Important Changes.