Using AC-21 for Job Portability

What you need to do is:
a) Look in your PERM or ETA750 for the DOL code.
b) Then go to
http://online.onetcenter.org/crosswalk/
c) Type in your occupation code.
(Eg: 039.162-014 will be the new 15-1021.00)

USCIS Guidance on AC21 is:
http://www.uscis.gov/files/pressrelease/AC21Intrm122705.pdf

Question 3. What is “same or similar” occupational classification for purposes of I-140
portability?
Answer: When making a determination if the new employment is the “same or similar” occupational
classification in comparison to the employment in the initial I-140, adjudicators should consider the following factors:
A. Description of the job duties contained in the ETA 750A or the initial I-140 and the job duties of the new employment to determine if they are the “same or similar” occupational classification.
B. The DOT code and/or SOC code assigned to the initial I-140 employment for petitions that have a certified ETA 750A or consider what DOT and/or SOC code is appropriate for the position for an initial I-140 that did not require a certified ETA 750A. Then consider the DOT code and/or SOC code, whichever is appropriate for the new position to make a determination of “same or similar” occupational classification.
C. A substantial discrepancy between the previous and the new wage.

Also
IV Spotlight Series on AC21