Mr Marr's mother Margaret Graham and stepfather Stuart will meet Solicitor General Lesley Thomson and Crown Agent Catherine Dyer later.
Local MSPs John Park and Willie Rennie, who lobbied for the meeting, will also attend.
Stuart Graham told the Courier newspaper ahead of the appointment: "It's a big step forward."
In his July 30 report, Mr Swindle recommended a review of fingerprint and forensic evidence, and the re-interviewing of five witnesses.
The Solicitor General is deputy to the Lord Advocate, head of Scotland's prosecution service. The Crown Agent acts as legal advisor to the Lord Advocate.