I am sorry to hear that you have problems upgrading you project.
You should not need to add any telerik assemblies in the GAC but the Telerik.Web.Design.dll. Resolving Telerik.Web.UI and Telerik.Web.UI.Skins references to the GAC can cause problems when deploying your project into production. So I recommend you uninstalling Telerik.Web.UI.dll and Telerik.Web.UI.Skins.dll from the GAC. Our installer deploy only Telerik.Web.Design.dll in the GAC in order to be accessible for all the projects in your machine for design-time work.
A possible reason for version inconsistencies in your project would be not upgrading all of the telerik projects in your solution. It is strongly recommended to upgrade all projects that have dependencies between them. Another reason could be having Telerik.Web.UI.dll and Telerik.Web.UI.Skins (v.2012.1.215.40 in you case) in the GAC.
Let me know if you think the reasons for the issue are different from the ones I mentioned.
Please do not hesitate to contact us if you experience any further issues.
the Telerik team
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