According to AC21[1] 106(c)

", the approval of a Form I-140 employment-based (EB) immigrant petition shall remain valid when an alien changes jobs or employers, if: • A Form I-485, Application to Adjust Status, on the basis of the EB immigrant petition has been filed and remained un-adjudicated for 180 days or more; and • The new job is in the same or similar occupational classification as the job for which the petition was filed. "

[1]Policy and Procedural Memoranda on the American Competitiveness in the Twenty-First Century Act of 2000 (AC21)

http://www.uscis.gov/USCIS/Laws/Memoranda/Static_Files_Memor...