Any idea on how AC21 works while changing employer when i485 has been pending for more than 180 days. I would like to know where would the process start with new employer once you switch the employer -ie Can new employer directly submit i485 with the old approved i140 or does it start with Perm approval. Any sajha users who has gone through this route or know how this works.

You need to submit a properly completed and signed Supplement J. Confirmation of Bona Fide Job offer or Request for Job Portability Under INA Section 204(j), showing that the offer of intended permanent employment as stated in the form I-140 immigrant petition continues to exist.

Please refer to the below USCIS links for the details.
https://www.uscis.gov/i-485supj

Disclaimer: This is not a legal advice. Please consult with your immigration attorney for the legal help.

Also the SOC code of the job should be similar to the one you left. Meaning the job should be similar at least in the paper.