Why do you want to do that? If it's a proper plaster (ie done by the spread rather than the stick on stuff) cornice it'll be virtually an integral part of the ceiling as it will have been done at the same time as the original ceiling.
If the old ceiling is in a bad state of repair then board over it and skim. Have to be clever matching in with the cornice though but it should be doable.
Taking a ceiling down is quite unecessary and a seriously fithy job.