One of the central ideas behind authority files is that they can help librarians keep their databases organized and ensure that all books about the same subject are described with the same subject headings. In order to keep the authority file useful, headings must be updated to match current linguistic usage (for example, the Library of Congress changed the subject "Cookery" to "Cooking" since no one would ever search for the former). Unfortunately, Koha will not automatically update bibliographic records using a given authority. There should be a flip_heading.pl utility that converts obsolete headings in bibliographic records to the current preferred form (as specified by the authority record).
When changing the main heading of a linked authority record, the records are upadated automagically.
See AuthorityMerge* preferences.