def METHOD_NAME(self, old, new):
    if old is not None:
        old.observe(self._update_value_mapper, "updated", remove=True)
    if new is not None:
        new.observe(self._update_value_mapper, "updated")
    if old and new:
        if new.range is None and old.range is not None:
            new.range = old.range
    self._update_value_mapper()