L1B Visa to H1B Visa Conversion

Question

Hi Saurabh,
I am working for company A on L1B and L1B is going to expire in May 2013.(I-94 valid till Jan 2014)
And I got my H1B applied and approved by company B this year.

1) Can I continue to work on L1B with company A till April 2013 and start to work for company B under H1B then onwards?

2) Can I go to India for vacation on L1B in Dec 2012 and return back on L1B in Jan 2012, continue working for Company A till April 2013?

3) If I go to India for vacation, is it mandatory to get my H1B stamped. Or can I return on L1B and get H1B stamped in my next trip.

4) If I go to India on L1B and return on L1B, is my H1B still valid?

5) Is it mandatory to start on H1B from Oct 1st?

 

Thanks in Advance