Transfer from L1B to H1B with COS after october 1st ?

I am currently in US on L1B with Employer A, I applied for H1B COS with employer B and got through the lottery. and got RFE, I have responded to subsequent RFE request and status showing still pending. My question is
will L1b o H1b change of status happen immediately on the day of the approval of the case or is there any grace period to join the employer B.

How soon I need to join the new employer B?
Due to some personal reason, I would like to continue with Employer A for another 6 months. How to avoid the change of status with leapfrog option?
or is it possible to avoid the change of status, while USICS status is still pending?
Thanks in advance.