Answer
Under the American Competitiveness in the Twenty-First Century Act (AC21), you can "port" your green card process to a new employer if your I-485 has been pending for 180 days or more and you have an approved I-140. If you are laid off before the 180-day mark, you are in a more precarious position. However, if your employer keeps you on the payroll via severance, your official termination date (the date you no longer have a "bona fide" job offer) is what typically triggers the legal concern. You must find a "same or similar" occupational role to maintain the validity of the underlying green card petition.