In one of the applications I was working, I noticed that as we have developed the application we have used both System.Xml and System.Xml.Linq namespaces. Now this can happen for a number of reasons including that as the application got developed, the new namespaces were used but the old code has not been refactored and it's not a priority as well as the latest .NET framework supports it. However, sometimes we might need to convert files from old to new version and vice versa. In my situation, I had to convert XDocument to XmlDocument and vice versa. To cater for this requirement I wrote a class with static methods that perform the conversion taking advantage of the XmlNodeReader and XmlReader classes.
Here's the sample code of the class.