This is what I did.

As the response for the NIW 140 RFE, I wrote a letter and sent it back with the RFE. In that letter, I told USCIS that I wanted to voluntarily withdraw the NIW 140 application (case number) since I already had another I-140 approved, and I would like to ask them to help me transfer my I-485 (case number) to the approved RIR I-140 (case number).

That's it. I did not file another I-485.

Hope this helps.