Thank you ykhan. My I-485 was approved yesterday. I changed job in February. My new company lawyer advised me not to send anything to USCIS since it would only raise a red flag. Then this thread further encouraged me not to do so.
One thing - ABSOLUTELY make sure that your old employer won't send I-140 revocation. In case you hired your own lawyer, it won't be a problem. But if the company financed the process through their laywers, they have the leash. And there's a risk. In my opinion, whether the old employer files for withdrawal or not is even more important whether you file AC21 or not. Even if you parted on good terms, you don't know what they will do. That's what happened to me, almost. A long story. Even if your I-485 has been pending for more than 180 days, etc., the revocation might trigger a slew of problems. There are more people involved you might not take into account - for example, the company lawyers (in my case, the damn Fragomen) will definitely raise the revocation option to the HR. So just be careful and don't trust anyone!