This is a good question, and one that not many federal contractors take the time to consider. In this situation, OFCCP would be interested to know at what point the opportunity for diverse outreach was provided. If the process for selecting the contractor was competitive and followed the regulatory requirements for posting, then the conversion, without competition is less problematic. The safest route, however, is to go ahead and post the position as you would any other opening.
You can use this OFCCP audit checklist to ensure you're doing what is required to maintain OFCCP's regulations including VEVRAA, Section 503, and EO 11246. Or request a demo to streamline your compliance and recruiting efforts.