Employer will terminate I-140 and you will have to start all over with the new employer.
NYC EXPERIENCED IMMIGRATION ATTORNEYS www.myattorneyusa.com; email: email@example.com; Phone: (866) 456-8654; Fax: 212-964-0440; Cell: 212-202-0325. The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter.
The biggest risk is that, as you said, you need to start afresh.
The second risk is that the current employer may blackball you in the community for not being a loyal employee.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
You should file premium processing on the 140 and get it approved before you change employers. Once the case is approved, even if the employer cancels the petition, you can still use the date to port to another green card.
Business Immigration Attorney. For H, L, J, EB5s, PERM and EB1/2/3 Petitions. Call 800-688-7892 or visit www.ImmigrationDesk.com. Law Office of Anu Gupta. The advice suggested here is for general information only and not to be construed as legal advice.