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Well, you have it a little bit confused. It isn't when your I-140
has been pending for 180 days or more that you can switch employers. It's when your I-485
has been pending for longer than 180 days that you can switch employers to the same or similar job and not have to file a new Labor Certification and new I-140. Since I assume you cannot file the I-485, any new employer, IF you want to get a green card
from them, would have to file a new Labor Certification and I-140. That's the bad news. The good news is that you do not lose your place in line. You get to keep the original Priority Date from your first I-140 even if your original employer withdraws it. Of course, if your original employer does not withdraw it and your PD becomes current, you can always file an I-485 at that time and when approved go work for that original employer for at least 6 months. If you don't want to work for that employer, then any new employer would have to file a new LC and I-140 for you to get a green card through them. But like I said, you generally would keep your original PD.
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