If you are talking about the RadEditor Web Part, then the answer is no. We cannot replace the original content editor web part because it is built in such a way that it only works on Internet Explorer. If we want to maintain full browser compatibility, then we must use a separate web part.
However, for the list and WCM scenarios the answer is yes. Should you decide to go back to the original rich text editor, you will not lose your content. In the case of the list field, you just need to disable the RadEditor features from the Site Features page. In the case of the WCM publishing field, you need to do a global Find and Replace from SharePoint Designer and change the tags to the original editor. Even if you have many pages with RadEditor, you can still revert to the old one with just one operation (find and replace).
All the best,
the Telerik team