How to delete old obsolete terms after node's term field has been updated?
A better description of what you're looking for is "orphaned terms". There exists a module that addresses that use case, called Delete Orphaned Terms.


  This is a utility module for deleting "orphaned" taxonomy terms. By
  default, terms with zero nodes associated with them are considered
  orphans, though administrators can specify the number of nodes
  associated with a term in order for it to be considered an orphan.