USCIS手冊說的很清楚

USCIS expects您換工作後，要主動交AC21申請

1) 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.

如果您不交AC21， 原公司有撤除了I-140 sponsorship, 移民局一定要issue "Notice of Intent to Deny"
2) Accordingly, if the underl ying approved Form I-140 is withdrawn, and the alien has not ted evidence of a new qualifying offer of employment, the adjudicating officer must issue a Notice of Intent to Deny the pending Form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. See 8 CFR 103.2(b)(16)(i).

這時候，你必須在30天以內證明你的新公司和原來的I-140申請公司性質一致.
If the evidence of a new qualifying offer of employment ted in response to the Notice of Intent to Deny is timely filed and it appears that the alien has a new offer of employment in the same or similar occupation, the USCIS may consider the approved Form I-140 to remain valid with respect to the new offer of employment and may continue regular processing of the Form I-485.

如果你30天以內沒有回應，或者新公司不是和舊公司工作性質一致，移民官要立即deny AOS申請人的I-485
If the applicant responds to the Notice of Intent to Deny, but has not established that the new offer o f employment is in the same or similar occupation, the adjudicating officer may immediately deny the Form I-485. If the alien does not respond or fails to timely respond to the Notice of Intent to Deny, the adjudicating officer may immediately deny the Form I-485.

所以， 從保險起見，換公司，還是file AC21為穩。