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.
Unless the context shows otherwise, all answers here were provided by Rajiv and were compiled and reported by our editorial team from comments and blog on immigration.com