Agree with “tammy2” many have filed I-485 without past employer letter with I-485 and got EAD, AP. Once I-485 is accepted don’t shout keep quiet for 180 days after filing I-485, don’t tell your friends they mat leak the info to past employer and employer may attempt to revoke I-140. Play safe.
Once you file I-485 they just look for your copy of I-94 if you are in status, give a look to I-485 form, correct fees and you attach the copy of I-140 filing receipt or copy of approved I-140 and you are all set.
** before filing please check the status of your I-140 from BCIS online status if it shows approved immediately take the print out and keep with you as a proof and file I-485, EAD, AP if I-140 is approved.
Many have done this and I-485 is taking 24-30 months at this time and when they are going to approve I-485 they will check medicals, birth certificates , employer letter and you will cross 180 days and can send new employer letter and can use AC21. many lawyer will refuse to file I-485 without employer letter go to lawyer who is ready to file and inform him you are ready for the risk or you file directly.