Your attorney should be able to guide you on filing an I-485J. It’s a fairly straightforward process. Are you looking to use AC21 to change jobs while your AOS is pending? If yes, your new employer’s attorney can file it for you after you join them.

thanks @A K. Since I was on L1, max out as of now, does my existing employer play any role and has any say in the supplement filing process.

are you with the same employer who sponsored your I-140 & I-485? With the existing EAD+AP and the same employer, you only need to inform your HR of your switch for work authorization from the L1 visa to the EAD. with a new employer, you’re essentially exercising AC21 portability, so they would need to file it. there’s no USCIS filing fee for them, only attorney charges. you would have taken up this new employment only with your EAD.

thanks @A K appreciate your response