I saw that at several places.

One of them is in www.immigration-law.com under either breaking news or Advanced Q&A. The basic step is pretty simply. Write to the service center where you 485 is processed about your request for substituting underneath 140 and using the PD of earliest one as the PD of 485. You need to supply 1) receipt of 485, approval notice of both 140s and specify which 140 you want to use.