The TreeView should be able to figure out the Old / NewValue for its event args during editing.
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Reason for deletion. In databound scenarios e.OldValue and e.NewValue refer to the same instance of the ViewModel which is expected. 

Saving the state of the ViewModels means creating an entire deep copy of the ViewModel. We believe there is no such control in the framework to save the states of its ViewModels during editing.

We have to admin, introducing public properties OldValue and NewValue of type objects for these event args is a bad design. This is a bug since these are public but we won't fix it due to the complexity (described above).