This 2003 Yates Memo provides guidance on processing Form I-485, Application to Register Permanent Residence or Adjust Status, when the beneficiary of an approved Form I-140, Petition for Immigrant Worker, is eligible to change employers under Section 106(c) of AC21.
…if an alien is the beneficiary of an approved Form I-140 and is also the beneficiary of a Form I-485 that has been pending 180 days or longer, then the approved Form I-140 remains valid with respect to a new offer of employment under the flexibility provisions of Section 106(c) of AC21.
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 Section 106(c) of AC21. It is expected that the alien will have submitted 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…
Read the full USCIS memo on I-485 validity (.pdf).