In general we have implemented many changes to the controls since the 2009 version (especially changes related to the design and skins). Therefore it is expected that you encounter issue. Unfortunately there is not a summary list with all changes but you can check the release notes (at the end of the page) ordered by date here
My best suggestion would be to open a support ticket where you can attach a simplified runnable project demonstrating the issue that you encounter, so we can inspect it locally and help you out in the most efficient way.
the Telerik team
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