Ever since Paramount bumped G.I. Joe: Retaliation to 2013, people have been speculating as to the real reason why. The studio offered up the transparent excuse that they were converting the movie to 3D, an excuse few were buying.
Deadline suggests that the real reason behind the delay is Channing Tatum, or at least the studio's decision to expand his role in the sequel. When the first G.I. Joe was released, Tatum wasn't a go-to box office draw, but with back-to-back successes in The Vow and 21 Jump Street, the studio decided to expand his role in the film.
Paramount was also extremely concerned about the sequel's worldwide box office prospects after test screening scores were mediocre to bad. Reshoots were planned, though director Jon Chu has been kept only partially involved in the decision-making process.
In fact, the bright spot for audiences during the test screening was the aborted relationship between Tatum and Dwayne Johnson's character Roadblock. It seems Paramount made a miscalculation in (spoiler alert that seems pretty obvious) having Tatum killed at the start of the film. That pretty much explains why he's been absent on most of the promotional material surrounding the movie.
Additionally, the film was slated to open up against another Channing Tatum movie, Magic Mike, a film directed by Steven Soderbergh and based on Tatum's experiences stripping, pre-stardom. Paramount didn't want dueling Tatum competing on the same weekend, especially since his fans were more likely to flock to Magic Mike rather than a film he was barely in.
Of course, another reason that hasn't surfaced yet, is the release of The Amazing Spider-Man, which is trending for a solid opening and would have potentially K.O.'d any fanboy momentum Retaliation would have picked up on opening weekend.
The move to March 29, 2013 pits the film up against the latest Stephenie Meyer film adaptation, The Host as well as another Tyler Perry movie.