L1B to H1b COS - stamping and effective date

Hi,Im in L1B at the moment and have been in the country for 5 years maxing the current visa on sep 6th 2015. My company (A) applied for a H1B (FY 2016) as COS and it was approved. i797 should be received any time now. Since the current visa is valid only till sep 6th, my company gave me two option.File for extending my period of stay from sep 6th to sep 30th in US or arrange for my stamping outside USA.Pls help clarify with the below please :– Will i reenter with H1b? If yes, Is it possible to file for H1B transfer (To company B) even before oct 1st 2015?– Are there risks of rejection with going for stamping before H1B takes effect? Im planning to travel in june or mid july for this.Thanks,