Please read thread pertaining to AC21 portability act.
If your I 140 is approved and I485 is pending since more than 180 days, you can use AC21 portability and change job if it is same or similar.
After changing job you need to inform USCIS regarding change of job under AC21 portability. If your employer send application of revoking I40 before you inform USCIS, they will send you intent of deny. Than You have to prove that you have used AC21 portability.
The bottom line is even if employer revoke I 140, your I 485 will remain valid as long as you qualify for AC21 portability
Make sure; your I 140 approved, I 485 is pending since more than 180 days and new job is similar/same as of your labor certificate discription