You must change your job after at least 180 days of filing your I-485. Otherwise you risk of losing grant of GC. Stay at the company who petitioned I-140 for you as long as you can. You have six months grace period for looking for a new job if the company is dissovled. You can renew your EAD annually. It has nothing to do with who you work for. It is the other way around, whom you will work concerns if you have a valid EAD.