No, you do not need to post those jobs. The posting requirement only applies when a company is hiring from the external pool. Since the interns are staying on the payroll and thus employees of the company, it appears that they would be undergoing a promotion or a transfer (from PT to FT perhaps). Positions filled internally do not need to be posted for external pools.
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.