L1B to H1 conversion and H1B extension

I have been working on L1 since 5 years. This year I got my H1B approved and it is valid till Sep 28, 2013. My lawyer says she has already filed my status of change which will be applicable from oct 1, 2012.

  1. How many days I have to change my status from L1 to H1? Do i immediately have to be on new Emplyers payroll from october 1 who is holding my H1?

  2. Can I transfer my H1 to some other employer instead of joining this employer who filed H1 for me? Or Do i need to be on his paystubs for sometime before I change my employer?

  3. As i understand, in order to get my next extension for H1, I will have to start my GC processing otherwise I will not be able to get any extension for H1 since I am already completing 6 years next year. I wanted to know how much time before the expiry of H1 do I need to start this processing of Green Card so that I cna get EAD or next H1 extension?

Thanks so much for your time and effort in answering our queries.