GC is a future job offer and does not require one to be working for the gc sponsoring employer when the employer starts the GC process.
Your h1 status ends the moment you start using EAD to work for another employer. You can always get back into H1 status but wld need to re-enter the country with a valid H1B stamp - Any employer is fine.
It is not possible to switch from EAD to H1B or any non-immigrant status in the US.
AC21 helps if you quit the gc sponsoring employer or if the employer closes shop.
This is not legal advice
Total Contributions to date:: over $750
PD Oct 2006/EB2/India
I140 approval Oct 2006
I485 filed Jul 2 07
I485 approved on Jul 25 2011
Received green cards on Jul 28 2011