There is no problem for you to have different versions of Telerik.Web.UI in the GAC. This is the whole purpose of the GAC (to be able to have different versions). What's important regarding SharePonit is using the fully qualified name when referring the assembly. This includes the pages and the web.config file/files of your SharePoint site.
Please review the articles, which I provided you with in my previous post:
Deploying RadControls for ASP.NET Ajax
Incorporating RadControls for ASP.NET Ajax in SharePoint WebSites
If you have any question regarding the deployment, or usage of Telerik.Web.UI, I will be glad to answer.
What is important in short terms is:
1. You must have the Telerik.Web.UI assembly in the gac.
2. You must have the SafeControl elements in the web.config file. Fully qualified name is mandatory here.
3. You must use fully qualified name when referring Telerik.Web.UI in the page.
4. If you decide to update Telerik.Web.UI.
You will need SafeControl elements for all previously used versions of Telerik.Web.UI and a binding redirct element in the web.confing (in point 4. of the first article you can find a sample dependentAssembly element). It will not be mandatory to change the version if the page references.
I hope this helps.
All the best,
the Telerik team
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