I-129 is company's A petition - -you are just a beneficiary in it. Company B needs to file a new petition. This is setting aside the moral aspect of you acting dishonestly toward company A
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You are assuming the currently filed H-1b will be accepted and approved. Both are unsafe assumptions.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
No, you don't transfer your H1b to an employer whom you never worked for. Instead, the new employer files a new H1b based upon the originsl I-94 provided the EAD is still valid. If expired, or out of status, then you 'may' have to consular process the second H1 B visa petition without a change of status.
I strongly recommend an appointment with a competent and experienced immigration and visa attorney before you do anything more. Good luck.
This is general information, not legal advice, and does not create an attorney client relationship.