Why was the definition of chiral carbon changed?
The IUPAC definition of asymmetric carbon is still:


  The traditional name (van't Hoff) for a carbon atom that is attached to four different entities (atoms or groups), e.g. Cabcd.


The definition of chirality centre is:


  An atom holding a set of ligands in a spatial arrangement which is not superposable
  on its mirror image. A chirality centre is thus a generalized extension of the concept
  of the asymmetric carbon atom to central atoms of any element, for example N+abcd,
  Pabc as well as Cabcd.