i140 H1B extension is required if you have completed your 6 year quota on H1B. I-140 petition is the 2nd step in a 3-step process for employment based green card.
I-140 in progress, H1B transfer to new employer B and start working?
You can file H1B transfer while your i140 is pending with USCIS.
I-140 petition (thereby your Green card) is for a future job with Employer A and hence, unless I-140 is explicitly revoked by your Employer A (current employer), the petition remains valid and will reach a decision stage some day.
You can do H1B transfer and can start working with Employer B anytime during your i-140 is pending.
I-140 in Progress, H1B transfer done, old Employer revokes I-140
I-140 revoked before approval – You cannot use i140 if it has been revoked by employer before its approval.
I-140 revoked after 180 days of approval – You can use the approved I-140 to maintain your priority date with new Employer B by filing new PERM and I140.
I-140 is approved and NOT revoked by Employer A: You can use the approved I-140 to maintain your priority date with new Employer B by filing new PERM and I140.
You can join Employer A as and when your Priority date becomes current and file your I-485 for green card.
I-140 approved, H1B Transfer to new employer B
Your employer A, can revoke the I-140 if 180 days have not passed from the date it was approved.
After 180 days, Employer A, can still file for revocation but USCIS may not revoke it, unless they consider it a FRAUD case.
You can use your I-140 priority date to port to new employer B.
I-140 approved, H1B transfer, I-140 transfer to new Employer B
I-140 is tied to employer and job. It cannot be transferred. You can PORT the priority date to new employer B, though.
You can also use approved I-140 from old employer to apply for H4-EAD.
I-140 approved, H1B Transfer and H1B Extension beyond 6 year?
Yes. Approved I-140 can be used by any employer to extend your H1B status beyond 6 years.