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AC21 law bsically protects your I-140 if you change your employer. Your employer cannot withdraw I-140 filed in behalf of you 180 days passes, basically I-140 becomes yours.
485 applicants can change jobs even prior to AC21 law, only thing is your former employer can withdraw I-140 filed on behalf of you. INS have no problems as long as there is a employer who offers a job after your GC approval.