You can continue using the old version of the Report Designer as long as you need to. About the message, do you mean that the Upgrade Wizard is invoked every time the solution is loaded? If this is the case you can disable the automatic Upgrade Wizard starting with the checkbox at the bottom
Still have in mind that due to VS limitations we are able to register only the latest version of the reporting toolbox items. This means that if you need the reporting toolbox items you will have to uninstall the newer Telerik Reporting installations.
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Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items