weiqunl ,
I changed my job 7 months after filing I-485 using AC-21. The legal department of my old employer(they are not lawyers,folks experienced in immigration matters) told me I do not have to inform INS unless they asked. This was based on the wording of the AC-21 rule, which could be interpreted to say informing INS is optional. Also there is no standard paperwork that needs to be filled for AC21.
However, being a person who worries a little too much, I contacted the legal dept. for my new employer. She said they have to inform INS. She made me get copies of the required documents(approved Labor petition.approval notice for I-140, I-485 receipt notice). Also she wanted an employment letter detailing my work and salary. I created one based on the labor petition(I repeated it almost word for word,even used the same title though the actual title with the new employer was different). She also got me to sign some more letters for INS and send it to her.
All these paperwork with my new employer was very slow.
I got my GC approved without any RFE though. I am not sure if these documents helped, but even if it was send(which also I suspect now,I am not planning on aksing them that) I think it reached INS around the time they approved my GC or after that.
It took the new employer nearly 6 months to get all the paperwork ready.
Hope this helps.
One of the questions I asked my legal department was that we seemed to be doing all these AC-21 stuff months after me joining them and wondered if the INS will not get suspicious. They said it does not matter.
I also used my EAD(completed I-9) and renewed it in between.
I worried a lot the whole time and even tried to hire a lawyer (who told me that they will be too expensive for me!).