I believe that if the new job is broadly in the same area as that defined during I-140, you should be fine. USCIS just wants to discourage people from making a drastic change in employment. For example, if you get an I-140 for Cataract research and become a McKinsey Consultant, its definitely not acceptable. But if you move from Cataract research to Retina Detachment research (hypothetically), USCIS should not have much problem with that. I hope you understand what I mean. Being a researcher, you are expected to evolve your research, so you have room to maneuver.