With visa backlogs and long wait times, clients are often afraid to change employers as they do not want to lose their priority date. Essentially, the priority date establishes your place in line waiting for a family or employment based green card application to be approved (http://classic-migration-sandbox-52046.hs-sites.com/green-card-faq).
In an employment based immigration case,(http://immigrationbn.hs-sites.com/blog/bid/93867/How-to-Qualify-for-an-EB-2-or-EB-3-Visa) the priority date is established through the approval of an I-140 immigrant preference petition. The actual priority date is the date the labor certification was filed. The beneficiary can retain that priority date for all subsequently filed I-140 petitions, irrespective of the employer, occupation, preference category, or location. If the I140 petition is denied or is revoked for fraud or misrepresentation then the priority date is not established. The USCIS, Adjudicator's Field Manual, at Chapter 22.2(b)(5)(A)(5), states that only a revocation based on a finding of fraud or misrepresentation results in a loss of the priority date. Therefore, even if an employer withdraws an I-140, USCIS will revoke the petition but the Beneficiary will not lose the priority date.
Priority dates are important when the beneficiary has to wait for a visa to become available. Wait times can range from a couple of months to several years. For example an applicant in the EB-2 category files a Labor Certification application on November 1, 2014 and an I140 application on July 1, 2015. If he is from Germany—the only wait time will be the time it takes USCIS to process his petition to receive permanent residency. If he is from India, the wait time is almost 9 years before he can file his adjustment application unless he has a previously approved labor certification filed in September 2008 in connection with a different I140 application. In such a case (as of the July visa bulletin) his wait time would be about a month before he could file his adjustment application.