H4 to H1B COS and then H1B transfer

Hi All,

I maybe completely misunderstanding this, but it will really help if anyone can guide me on my below stated situation.
-- I am currently in USA on an H4 visa with an LOA (Leave of absence) from my current employer A in India.
-- My H1b petition for 2013-2014 has been approved with employer B effective Oct 1st 2013.
-- Now I have received a better offer from an employer C who is ready to transfer my H1b this October, once it gets effective.

My questions are:
1. Do I need to resign from my currently employer A and join B to initiate the H1b transfer from B to C or just an approved petition copy from B suffices?
2. In either case, if C files for the transfer in October first week, will I need to provide pay-stubs from employer B for transfer to C?
3. Is there a time-limit starting Oct 1st in which I need to join the new employer and does my H4 status automatically cancels starting Oct 1st?

Thanks a lot for the help and guidance.