Under AC21, you can change jobs if you have a i 140 approved and i 485 pending for 6 months. Your new company can hire you and continue responsibility for your 485, which is not a big deal from what I know. I think you cannot change your field of work, but you can verify this with a lawyer.
If you have not yet applied for i 485 due to retrogression, you can still change jobs on h1b and you will get h1b extensions in 3 year extensions. You can apply for your new GC under PERM and retain your existing priority date because your i140 has already been approved.
Good luck