I don't have personal experience with the process, but yes all you need is the EAD to change employers after the 180-day period of pending I-485 has been completed. The lawyer will file a AC-21 package with USCIS which includes the I-485J. Trackkit has a sample cover letter with the document list to be included for AC-21 (all this the lawyer should know already):
https://www.lawbench.com/sample-docu...etter-for-AC21
For the I-485J to be applicable, the lawyer/employer needs to confirm that the new job description and responsibilities are similar to the underlying PERM of the original job for which the I-485 was filed. The SOC code match would be ideal but not necessary. Also job title match isn't necessary.
In summary, looks like the employer/lawyer are doing their due diligence. Not much else to do on your end, except reviewing all documents for correctness before they are mailed out to USCIS.