You can change your 485 from EB1B 140 to NIW 140 only when your PD is current based on the EB2 China cut-off date in Visa Bulletin. This is called "transferring (485) underlying basis". USCIS website has more details on the requirements and how to do it. https://www.uscis.gov/policymanual/HTML/PolicyManual-Volume7-PartA-Chapter8. html