Silverlight XAML and the new Win8.1/WinPhone8.1 XAML (aka XAML for WinRT) are two different things.
If you started your app with Silverlight you have to stick to it, or create your whole UI again with the new XAML (which as I said, it is now compatible with Win8.1)
Edit: Found an interesting article you might want to take a look at: http://msdn.microsoft.com/en-us/library/windows/apps/br229571.aspx
Edit See also this article: What's next for Windows Phone 8 developers (http://msdn.microsoft.com/en-us/library/windowsphone/develop/dn655121(v=vs.105).aspx).