When applying for a change to the CIP data for a book, one of the things they ask you for is the LCCN that has been assigned. In order to get an LCCN, you have to have received your CIP data, which means that you cannot submit a change application while a book's original CIP data application is still being processed.
I guess they've had problems with people trying to submit a change application early, however, since they feel the need to say this:
Provide only the Library of Congress Control Number from the lower right-hand corner of the original CIP data. Do not submit a change request using a made-up LCCN.