I changed my field from biology (at Louisiana, 2002) to environmental sciences (California 2004), and then to genetics (2005, genetics) and now environmental toxicology (North Carolina). I did get a RFE for independent evaluation for my I-140. I ted two letters from people working in government agencies and I-140 got approved three days later after I mailed it out (online information update). No RFE for I-485 (approved couple of weeks ago). Good luck to you.