I think it is not possible to do this in the Configuration file itself.
In some parts of Symfony it is also done in the extension.
https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php#L166
But i could turn around the logic, that the old configuration is only tried to pick up when the new is not set.