In order to have different projects with different versions of RadControls for WinForms in them you just have to keep your older versions of the controls installed on your machine. This means that when you are installing a new version you should not uninstall the older ones. If you are using our Control Panel in order to install new versions you should check the "Keep Previous Parallel Versions" check box in it so that you could keep your older versions (this your older projects will work).
If you have installed the newer version without checking this check box this means that the older versions have been uninstalled and this is the reason for your not working old projects.
In order to get them working again you should install again the older versions of the controls which have been used in your projects.
Let me know whether this helps.
All the best,
the Telerik team