The release dates for films are usually switched around up to nearly two weeks before the release of the film, in the case of G.I. Joe: Retaliation. Although it is a bit weird when a major film get changed after so much press has gone into their release date.
This has happened with Dawn of the Planet of the Apes and X-Men: Days of Future Past. We have known for a while that X-Men and Apes would be released in July and May of 2014, respectively. In an announcement yesterday by Twentieth Century Fox, they plan to switch the release date of both films.
Now, X-Men: Days of Future Past will be released on May 23rd, 2014, which puts that two weeks after the release of The Amazing Spider-Man 2. Dawn of the Planet of the Apes will get the July 18th release date.
No reason was given for the change in release dates. You can’t even pin it on production issues because two months is hardly enough time to compensate for any type of delay.