AC21 lets you port your in-flight green card application to another employer pre I485 approval. You need to wait 180 days after your I140 has been filed to switch employers using AC21 (assuming you have not yet been approved for your I485). Once your green card has been approved, this is no longer relevant.
"After that there is a mandatory 180 day period before you can switch jobs."
This is false. There's a lot of misinformation on the internet about this. Speak to an immigration attorney and you will discover there's no mandatory period whatsoever.
See also: Rights and Responsibilities of a Green Card Holder (Permanent Resident)
[1] http://travel.state.gov/visa/bulletin/bulletin_6028.html