Currently I work for company A, and got an offer from company B during last week of March.

Both companies filed for my H1B, and company B has a start date of May 1.

I talked to immigration attorney in company A (current employer), and they would not withdraw the H1B petition even If I terminate the employment.

If I move to company B on May 1, and only H1B of company A gets picked. Can I transfer it on Oct 2 to company B ? If that transfer fails, can I get back on F1 visa from H1B visa ?