3 to 4 months?.. Unfortunately, it won't be that easy. Even assuming that RD12/18/01 will be approved in 3 to 4 months, you would still have to endure another 6 months with the sponsoring employer just to make sure that INS will not see anything fraudulent about your Green Card in the future. Which totals to 9 to 10 months...
However, my advice is to stay with the sponsoring employer for as long as it needed. Bringing in AC21 may unreasonably complicate your case.
what do i need from current employer/attorney ?
Not much, really. You want to make sure that they won't withdraw their I-140 petition which they have filed for you. This should be done gently. If you do not part with them on good terms, but hint that they can make you sorry by just doing that, they most likely will.
While it is still unclear if I-140 withdrawal will have any serious impact on AC21 cases, if at all, it is better to be on the safe side and not to try your own luck. Try your luck behind a poker table, in Vegas, not with INS.
what do i need to let new company know ? and what do i need from new company ?
You will have to let them know that you are in a process of getting your Green Card and what you need from them is a permanent job offer.
You may wish to explore a possibility of transferring your H-1B to them instead of going to work on EAD.
what do i need to let BCIS know and how ?
Although there are different opinions whether to notify INS about using AC21 or not, I prefer to be conservative. So, I say - "Notify INS". A notification letter sent with certified mail will provide you with a proof that they received it, but it may never be matched with your file, which might be not that bad at all.
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Also you might want to visit
I-485 Portability (AC21) forum, which specializes on AC21.
