H1 transfer

I am working for company A in India. From Company A's USA branch I have a H1B visa.
I have an offer from Company B in US and they are willing to start the Visa transfer process.
Would I have to get a new stamp? What are the implications of my visa being rejected at the time of second stamping?
Also I have worked in the US for company A on an L1 Visa for a period of 30 months. Would this period be added to the 6 year stay limit in the US?

We have a few immigration experts who float through here from time to time, but not recently.... This is the type of question that should be answered by an expert so if you're still looking for an answer I suggest you consult an immigration lawyer.