So question on H4 and I-140, what happens if the H1B holder, having an approved I-140 (over 180 days), switches job. What happens to the H4 EAD?
Can employer A withdraw the approved I-140 resulting in H4 EAD becoming unusable?
Will employer B need to file PERM and I-140 for H4 EAD to be active again?
Nothing happens to it. If it is over 180 days, they withdrawing does not matter, you can still carry the priority date and get extensions based on the first approval.
No, they can continue to work on H4 EAD as along as the primary H1B spouse is still on H1B and in US.
For using H4 EAD, they do not need to file PERM.
They will need to file PERM and I-140 during adjustment of Status, the key thing is that you will retain the priority date from first employer.
No need, they can use the old one that is approved for extensions.
This is done when the dates become current and you file for AOS. Your employer can do it ahead of time, so that you can go ahead for Adjustment of status, when dates become current.