The .NET framework is backwards compatible so you should not experience issues because of its upgrade.
The UI for ASP.NET AJAX version you are using, however, is very old and does not support any of the current browsers. It only goes up to IE9 and does not support mobile browsers or the current Chrome and Firefox versions, so you are likely to get issues on that front. Thus, I strongly advise you look into upgrading to the latest version.
Do you need help with upgrading your ASP.NET AJAX, WPF or WinForms projects? Check the Telerik API Analyzer
and share your thoughts.