Similar question in another thread, same page. One day we will have thread consolidation for the benefit of Q & A sessions. In any case, let us re-post the answer:
Once you let USCIS know about the change (AC-21 letter), and you fulfil the requirements for the I-140 approved, and I-485 pending for 180 days and the similar job requirements, nothing to worry about what the company can do. They have the right to try cancel your I-140 and/or re-use the LC for another candidate. That is their right. You may run across problems if you never sent the AC-21 letter and the company took some action wrt to the above (NOID or RFE or anything). Even in this case you can answer back with current docs. It may take a bit longer, right? Expect also a possible transfer in case of AC-21, which may take a bit longer also. Maybe not.......................................
You can use AC-21 as many times as you wish.
There are pros and cons about informing or not US CIS. Especially if the employer cancels or tries to do so, your I-140, or tries to re-use the LC for
somebody else (cases of fraud). The burden is on you all the time!
Transfers: Probably yes, if they know you used AC-21 in advance. This is in the adjudicator's discretion. The last memo about applicability of transfers was
in 1998, I believe...you can find it yourself.
Read the latest memos also about AC-21 and some more literature and decide yourself.
http://www.bls.gov/soc/socguide.htm
http://www.entertheusa.com/publicat...bility_ac21.pdf
http://www.entertheusa.com/publicat...weet_sorrow.pdf
http://www.murthy.com/news/UDportme.html Memo on AC-21 / analysis