If you already have your application working in WinForms, then it should be easy to convert the diagram-related functionality to WPF by continuing to use GoDiagram for WinForms. WPF applications can include WinForms controls, as the WPFusingGoView sample demonstrates.
Although a lot of the concepts are similar, there is little code that can be common between GoWin and GoWPF applications. (That’s true for our implementation too.) The main reason for this is that the GoObject/GoShape/GoText/GoImage/GoGroup/GoControl classes have been replaced by System.Windows and System.Windows.Controls classes. Furthermore, you expect to be able to really use data-binding in WPF or Silverlight applications – not just for showing details within a node, but also for links and for the whole diagram.
WPF might be Microsoft’s latest window system, but Windows Forms will remain supported for a long time. The same is true for GoWPF and for GoWin, and we expect to continue improving GoWin (and GoWeb) for a long time.
Summary: if you have something working with GoWin, keep using GoWin. If you are developing a new application, use either GoWin or GoWPF (or GoSilverlight) as appropriate.