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Yes, the plugin will take care to "upgrade" your existing content so it can safely be used with CKEditor

When you load existing content for the first time, the plugin will add the required divs with their classes so it can be edited and they will also allow you to more easily style the elements to get the look that you want for your website.

The plugin takes care of automatically import the styles that you have added in your contents.css (or whatever file you're using to style the contents of the editor), so that you get in your editor the same look that in the final page.