I applied my green card through my company... my I-140 is approved and
my I-485 is pending past 180 days. Recently I got a RFE to produce my
employment verification and I submitted it.
Now I am planning to change my employment by filing AC21. 
But I dont have my I-140 approval copy(it is with my employer, but he
is not giving me a copy).
Can I file my AC21 without my I-140 approval copy? OR If it is
mandatory, can I get a copy from INS using freedom of information act?
Need a detailed advise.

Hi balasez,

No, you do not need a copy of the I-140. The AC21 is not a form that
you file but rather a rule contained in Section 104(c) of the American
Competitiveness in the 21st Century Act of 2000 (AC21). It's something
you can take advantage of, not something that you apply for.  Although
not required by law, it's best to inform USCIS of your change of
employment by sending them two letters, one from you and one from your
new employer. Please click on the links below for full details.

Risk of GC Rescission for Failure to Notify Change of Job / Employer!
Posted Jan 28, 2005
"...This report is provided for those persons who may be considering
use of AC21, who are debating the issue of whether or not to inform
the USCIS of the job change. In this era, when in doubt, it is best to
follow the most cautious path of notifying the USCIS of any change in
job or employer to be eligible to obtain the approval of permanent
resident status and to maintain that status."
http://www.murthy.com/news/n_risres.html

Changing Employers When You're Being Sponsored for a Green Card
"Per the June 19, 2001 memo, you must supply the BCIS with two
documents. The first document is a letter, stating that, at the time
you submitted the I-485, it was your intention to remain with your
initial employer indefinitely. The second document is a letter from
your new employer stating that your new job is the same or similar to
the job for which your labor certification and visa petition were
approved. These letters may be submitted at any time, but the process
is much smoother if they are submitted before you receive any
notification regarding your application, from the BCIS."
"The employer letter should contain the job title, job description and
salary of the new job. To determine whether the new job is the same or
similar to the old job, the BCIS consults the Labor Department
Dictionary of Occupational Titles (DOT) or the online O*NET
Classification System (one of the two), or similar publications. The
memo prohibits INS officers from denying I-485 based on failure to
demonstrate that the new job is in the same or a similar occupation as
the initial job unless they have consulted, on a case-by-case basis
with BCIS headquarters."
http://immigration.about.com/library/weekly/aa090903b.htm 

"If an applicant for adjustment wishes to take a new job in the same
or similar occupational classification at the job that was the basis
of his or her employment-based I-140 AND the I-485 has been pending
180 days or more, the new employer may be substituted into the
existing I-485 application without disrupting the application at all.
This is accomplished very easily - NO new petition and no new fees.
Step 1: The applicant notifies INS of the change in intent by letter.
Step 2: The Service should then make a request for a letter of
employment from the new employer.
Voila! Done deal."
http://www.ilw.com/articles/2001,0705-Latour.shtm

Sample letters
http://boards.immigration.com/showthread.php?s=&threadid=80577

May 2005
Interim Guidance for Processing Form I-140 Employment-Based Immigrant
Petitions and Form I-485 and H-1B Petitions Affected by the American
Competitiveness in the Twenty-First Century Act of 2000 (AC21) (Public
Law 106-313)
http://uscis.gov/graphics/lawsregs/handbook/AC21intrm051205.pdf

The purpose of this memorandum is to provide field offices with
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 §106(c) of AC21.
http://uscis.gov/graphics/lawsregs/handbook/I140_AC21_8403.pdf

Additional Link of Interest

HOW TO MAKE A FOIA REQUEST
http://foia.state.gov/foiareq/foialetter.asp

I hope this helps. If you have any questions, please post a
clarification request and wait for me to respond before closing/rating
my answer.

Thank you,
hummer

Google Search Terms Used: ac21 I-140 "sample letter" uscis foia

good answer