Can I change employers during Green Card process?

Changing employers during Green Card process is complex:

• Before I-140 approved: Must restart process with new employer
• After I-140 approved (180+ days): Can use AC21 portability to change employers
• New employer must file new PERM and I-140 (or just I-140 if using AC21)
• Priority date may be preserved in some cases

AC21 allows: Changing employers if I-140 approved 180+ days ago and I-485 pending 180+ days.