I think I managed to change all our "cookery" authority records to their new "cooking" form. That means that, tonight, all subject headings linked to those records should change to "cooking" instead of "cookery."
This does not necessarily mean that all instances of "cookery" in our subject headings will be gone. Our Children's subject headings will probably continue to use "cookery" - since I only do authority work on Library of Congress subject headings and not Children's, MeSH, or anything else, the only subject headings that ever get updated are Library of Congress subject headings. Also, we have some Library of Congress subject headings in our catalog that don't have their corresponding subject authority records loaded yet - these won't change to "cooking" until I change them by hand and/or load their authority records and force the headings to flip.