Regarding H1 Transfer

I got approval in Oct2016 for the H1 B filing for company A. Now I came to know that the company freezed hiring and may layoff.

Can I transfer the approved H1 to another company (company B is willing to hire).

How does the process work? Does company B have to start from scratch or it is an easy transfer?

If company A allows me to come here in US can I stamp and change the job. Which one is preferable to change to company B, stamp and come or come on Company A and then change to B.

Please respond at earliest. Wonderful forum. Thanks