nxm, here is a little snip from immigration.com faq:
CHANGING EMPLOYER AFTER I-140 APPROVAL
If a person has received an I-140 approval through an employer, the priority date then permanently belongs to him or her. Under very limited circumstances (such as fraud) INS may revoke the I-140 thus causing a loss of priority date.
If such a person changes employers, their priority date will remain the old one, even though they have to process their labor certification and I-140 again with the new employer. It does not matter where in USA the new job is located, what the new job title is or whether the new job falls under EB-2 or EB-3. The priority date is still transferable.
We recommend that an applicant keep at least a copy of the I-140 approval notice.
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I believe that what you are saying about 180+ days are correct, however if your I-140 is approved, even though the new employer would have to go through the LC and I-140 again, you would still keep your PD. Think about it, having your I-140 approved means that both the labor department and USCIS agreed that you are fit for the position that you were applying for, so it is your right not to be all the way back of the line when you apply again.