First reported by the Post-Dispatch, Long's restructuring frees $4.5 million in cap space. St. Louis had just $244,000 in cap room before the move.
Long, who has 42 sacks in his five-year career, signed a four-year contract extension in the summer.
He's agreed to convert $6 million of his $13.25 million base salary this season to a roster bonus, spreading out the cap hit. He'll count $8.75 million against the cap in 2013.
According to Post-Dispatch beat writer Jim Thomas, the Rams are in better position to possibly re-sign safety Quintin Mikell or sign free-agent linebacker Daryl Smith, who visited the team three weeks ago.