By now, you should all know that LC's Classification Web is available for subscription. See http://www.loc.gov/cds/classweb.html for information about the product.

The best sources for subject cataloging policy and procedure are the subject specialists in the Library of Congress' Cataloging Policy and Support Office. People were wondering about LC policy regarding editions so recently I asked Paul Weiss of LC's Cataloging Policy and Support Office to reiterate their policy for us. He did and I quote:

Classification number. If a previous edition of a work was correctly classified and the number is still valid, assign the same classification number to the new edition. If the number is no longer valid, assign a currently valid number.

Form divisions. If an earlier edition uses a form division Cutter that is no longer valid, do not use that Cutter for a new edition.

Book number. If a work is entered under an author, and either the author or the title changes from one edition to another, maintain the same Cutter number for later editions that was used for previous editions.

If a work is entered under title and the title changes significantly from one edition to another, do not attempt to maintain the same Cutter for later editions that was used for previous editions.

Sometimes it is a nuisance to keep editions together since it means getting older editions from the stacks and reclassing them or adapting copy. It is more apt to be a problem with copy cataloging since FASTCAT staff or those staff doing LC copy may not be adept at recognizing editions or willing to do the work to keep editions together. Some of the copy catalogers may be trained just to accept call numbers without class number verification or edition checking.