H1B transfer to New employer while H1B extention is pending.

Currently I am with Company A and my H1B extension is pending from Company A from July 2012(Normal processing 4 to 5 months) in USCIS VT center. I got another offer from Company B and I accepted the offer and the Company B is filing my H1B petition tranfer under Bridging in Premium Processing.

My questions: What happens when My Company B petition approves first and then my Company A extesion approves after that? As per Last Action rule, do i need to go to India for Stamping in order to work for my Company B? I do not have H1B stamping in my Passport from Company A.

Thanks for your help.