H1b Transfer While Petition is Pending

[srinivas_gangam]

I have a new h1b petition filed by an employer A, It is still pending for approval. My current employer(outside US) B also wants me to work for them in US. As the quota is finished employer B wants to file an L1 for me. At what stage I can transfer my employer A h1b to employer B.

1. Can I transfer while petition pending and apply directly H1b with employer B instead of L1

2. Or can I transfer after approval and before stamping

3. Can i transfer after stamping?

Given the choice I would prefer to work for the current employer B in US. Please suggest how do i proceed.