I have used the AC21 portabilty rule to change my employer.
However I have not filed AC21 with the INS.
On some research on this in the forum, I thought the best is to not file for AC21 unless an RFE is issued on my case.
Now if AC21 is filed, INS would definitely understand that you have changed employer and so RFE would be issued to verify the new job description and pay stubs to verify that you are currently still employed. There is no drawback in filing AC21, but it could delay your case.
There is no rule as per AC21 that you need to inform INS within x days of your switching employers. You can file for AC21 in response to the RFE issue incase if INS finds out that you changed employer (maybe through your tax returns).
My recommendation would be to not file AC21 incase you are on good terms with your employer. I spoke with my employer (who filed my GC) and their lawyer (currently on record for my GC). They have no issues with me even though I left them since I have been working for about 4 years and since I got better offer, I asked them if they could match it. Else let me go. So they agreed, and my GC process stays. In case an RFE is issued, I would use the same company lawyer (ofcourse pay lawyer directly) since he has my case papers rather than switching to another lawyer.
But if your existing employer is furious filing AC21 would be a good choice. Because once you leave him, he will revoke your I140 and so anyway INS would know that you changed. But then the employers know that they can do nothing about your GC process, but still they revoke to trouble you and scare you.
Make a decision. But as far as for documents, I think there is nothing specific format. It has to be a formal letter stating you gc case nos, and that you are changing employer using the AC21 rule. Also send in your old and new job descriptions with the letter.
