For a workaround to those master pages that are using the content placeholders for some needed content you will have to edit the template you want to put on a already created page in order to preserve the content at the places it is as described in this
forum post. The template will be editable you will be able to drag widgets and edit them. The bad thing is you will get a visual at top of the content of your content placeholders placed on the current template, but that will not be visible in the front end.
All the best,
the Telerik team
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