Re-implement the setData method of the model to get the old and new values during editing:
class TreeModel(QtGui.QStandardItemModel):
def setData(self, index, value, role):
if role == QtCore.Qt.EditRole:
print 'old:', self.itemFromIndex(index).text()
print 'new:', value
return QtGui.QStandardItemModel.setData(self, index, value, role)