Change of status
1. Go as quickly as you can. It isn't an emergency, but it is something you should not take lightly.
1. The employer can revoke, but, USCIS says, after 180 days they will not revoke the H-4 EAD.
2. You will not need to change the H-4 EAD upon moving to any number of companies, as long as you maintain your H-1 status.
According, to USCIS, when you are working under AC21 you are not out of status, you are in fact working according to the law and hence you should be able to join back. More...