Hello fregas baratis,
Unfortunately, you cannot change the controls on your own, since some of our assemblies are built with the reference to the files of older versions. However, you can update to RadControls for AJAX which are in one single assembly (Telerik.Web.UI.dll) and they can coexist with the old RadControls therefore. Sitefinity will still be using the old controls (3.0 at least, we are upgrading to latest service pack of RadControls for AJAX in our Service Pack 2), but you will be able to use new controls in your own projects.
I hope this clears the situation a bit. Let me know if we can help you in any other way.
the Telerik team