The move saved the Lions more than $4 million of cap room.
Kitna, 36, was happy enough with the idea of ending up in Dallas as Tony Romo's backup that he reworked his contract to give the Cowboys more cap room.
Kitna added an undetermined number of years to the deal, according to a source, but he still will earn the $4 million.
Henry, 32, who had been with the Cowboys for four years and was a starter for the past three, was surprised by the move, according to a source. Henry was scheduled to make $3.6 million, including a $1 million roster bonus that was coming due.
Last season, Henry had 55 tackles, two sacks, two pressures, seven pass breakups and one interception.
Kitna finished the 2008 season on injured reserve because of a back injury. Thanks to escalators in his Lions contract, he was scheduled to make $3 million in salary, and he also had a $1 million roster bonus due early next week.
Kitna lost his starting job last season in Detroit, starting just four games, completing 68-of-120 attempts for 758 yards with five touchdowns and five interceptions. In 2006 and 2007, he threw a total of 39 touchdown passes and completed more than than 62 percent of his passes.
John Clayton is a senior NFL writer for ESPN.com. Information from The Associated Press was used in this report.