AC21 memo for switching jobs

http://www.uscis.gov/graphics/lawsregs/handbook/polpromem.htm

Look for Yate's memo on 08/04/03:

Continuing Validity of Form I-140 Petition in accordance with Section 106(c) of the American Competitiveness in the Twenty-First Century Act of 2000 (AC21)
08/04/03 William R. Yates /s/ Janis Sposato HQUSCIS

In that pdf file, you will read:

Accordingly, if the employer withdraws the approved Form I-140 on or after the date that the Form I-485 has been pending 180 days, the approved Form I-140 shall remain valid under the provisions of §106(c) of AC21. It is expected that the alien will have ted evidence to the office having jurisdiction over the pending Form I-485 that the new offer of employment is in the same or similar occupational classification as the offer of employment for which the petition was filed. Accordingly, if the underlying approved Form I-140 is withdrawn, and the alien has not ted evidence of a new qualifying offer of employment, the adjudicating officer must issue a Notice of Intent to Deny the pending Form I-485.