L1 B to H1 Change

Hi Travelled to US on L1 B from company A on Sep 2014.Applied for H1 in 2015 from company B got picked in lottery. But, got RFE and Visa approved in Dec 2015.got i-94 validity date from Dec 8th. what is the gap/ days before I join company B? Do I need to join immediately?Received a new I-94 which has a effective date of Dec 8th, 2015. Is my old I-94 (from my L1 entry time) is invalid ?Please share your valuable thoughts. Thank You.

You are on H-1 now as your 797 was approved w/ I-94 attached. Stop working on L-1 and start working on H-1 ASAP.