Almost the same time when my LC based I-140 was approved, I got an RFE for the NIW 1-140 with many challenging questions. I decided to give up that application.
I wrote back to the USCIS, in my response to the RFE, to tell them that I want to voluntarily withdraw the NIW 1-140 application because I have another I-140 already approved. I gave them the approved I-140 receipt number, and asked them to transfer my I-485 from the NIW case to the approved I-140.
They mailed me back a letter confirming the withdrawal, and my NIW I-485 was successfully transferred to the approved I-140.
You can try to do the similar steps. Make sure that you write clearly and support your letter with the copies of all necessary documents (especially the approval notice for the I-140).
Wish you the best luck!