Change the location of the RadControls folder? SOLUTION
Due to legacy issues, you might prefer one or more of the applications to stay with the previous versions of RadEditorPlaceholderControl. As the control searches its RadControls folder by using a virtual path, you can copy the older version of the RadControls folder to your desired place (let's suppose the root of your web application), open the Internet Information Services Management console, navigate to your web application, expand it to the CMS virtual directory and create a new Virtual directory under it, named RadControls, which points to the location of the RadControls folder you just copied. It is important to remove the application from the newly created Virtual Directory, because as the dialogs of RadEditorPlaceholderControl need to run in the same context, leaving it as an application would cause them to throw exceptions. The removal of the application can be made by opening the Properties pane of the Virtual Directory and choosing the Remove button beside the Application Name textbox in the General tab.
Of course, the location of the RadControls folder could be changed by using the RadControlsDir property of RadEditorPlaceholderControl, but then you will need to set this property for every instance of the RadEditorPlaceholderControl.