using System;

using System.Xml;

using System.Xml.Linq;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var xaml = @"<StackPanel>

    <TextBlock Text='not example'/>

    <!-- Some other stuff... -->

    <GridViewColumn Header='example'>

        <!-- Some other stuff... -->

    </GridViewColumn>

</StackPanel>";

        var templateXaml = @"<DataTemplate>

           <TextBlock Text='example'/>

       </DataTemplate>";

        var doc = XDocument.Parse(xaml);

        var template = XElement.Parse(templateXaml);

        //XNamespace d = "http://schemas.microsoft.com/winfx/2006/xaml/presentation";

        foreach (var attr in doc.Descendants().Attributes("Text").Where(o => o.Value != "example"))

        {

            attr.Value = "example";

        }

        foreach (var element in doc.Descendants().Where(o => o.Attribute("Header") != null))

        {

            element.DescendantNodes().Remove();

            element.Add(new XElement(element.Name.LocalName + ".HeaderTemplate", template));

        }

        Console.WriteLine(doc.ToString());

    }

}