L1b to H1b

[Roselyn]

I'm currently on L1b visa and started working for employer A in US from November 2015. I already have an approved H1b (employer B) from last year and would like to use it. But as my h1 visa was filed while I was not in US, change of status is not applicable in my case as far as I know. In this case, to work for employer B on h1 visa do I need to go out of country and get visa stamped? If I do not get visa stamped can I return to US on already valid L1b visa?

Please share any information you have on this situation.

Thanks