While Detroit frees up cap space with the maneuver, Stafford's money is now fully guaranteed regardless of any cancellations during the season.
Stafford had already agreed to restructure his 2020 salary last December, freeing up $4.8 million at the time.
Per Spotrac, the quarterback has a $21.3 million cap hit for the upcoming season.
The 32-year-old missed the second half of last season with a back injury, although he had been off to a strong start in the first eight games.
Stafford had 19 touchdowns and just five interceptions, producing a career-high 106.0 quarterback rating. His 312.4 yards per game were the second-most of his career behind only the 2011 campaign when he topped 5,000 passing yards.
The injury was also a change for a player who had started every regular season game over the previous eight years.
He is now seeking a healthy 2020 campaign while helping the Lions bounce back from a 3-12-1 season. The extra cap space could help the team fill out the roster and create a contender in the NFC North.