What is different if you change employer before or after 180 days from I-140 filing? The only difference is that employer can revoke your application if you move before 180 days and it cannot after 180 days?
Yes unless your new job description is drastically similar in paper and salary and role. If you are from rest of the world, it would be wise to stick until you get GC. Else, it doesn’t matter. You have to wait 10+ years anyway.
Yes. But you retain your priority date
Approved i-140 only gives you a priority date and an underlying basis for filling adjustment of status. If you leave your employer you lose the underlying basis but may keep the priority date. A new employer will have to start the green card process from the PERM stage even though your new PERM will not dictate your priority date.
PERM process has to be started from beginning anytime you switch employer. 180 day period with same employer whom you are currently working with helps you to retain your priority date. You'd loose priority date and your new i140 approval date will become as your priority date if you switch before 180 days.
Correction: Perm file date would be the priority date.
I am not sure the above (@New’s comment) is accurate. My understanding is you would lose priority date only if previous employer revokes the I-140 (they can do that if you switch before 180 days however they have no incentive to do this). Happy to be corrected.