Hi Kevin,
The error you are currently facing indicates that there is a mismatch between the loaded dll assemblies and the ones that the project is initially build with . Please refer to the following topics where possible solutions to this problem are described:
http://www.telerik.com/help/aspnet-ajax/introduction-troubleshooting.html
Design-time Troubleshooting
http://www.telerik.com/help/aspnet-ajax/introduction-troubleshooting-skins.html
RadControls Trial Message
Could you please share whether you have tried the Telerik Upgrade Wizard?
http://www.telerik.com/help/aspnet-ajax/introduction-vsx-upgrade-wizard.html
I hope this will prove helpful.
All the best,
Eyup
the Telerik team
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