The Abrams signing was a shock to everyone given the director/producer/writer had publicly stated just a few months ago that he was not interested in directing a film in the Star Wars franchise given his attachment to Paramount's rebooted Star Trek. With Abrams on board, Lucasfilm is moving full speed ahead on their new Star Wars trilogy and first films under parent company Disney.
Lucasfilm's exact wording for delaying the prequel 3D releases reads, "we will now focus 100 percent of our efforts on Star Wars: Episode VII in order to ensure the best possible experience for our fans" after the Abrams' signing was made official. I suspect Lucasfilm also wants to keep the prequels out of the spotlight and young viewers minds as they go in a new direction for Episode VII, or at least what should be a new direction that in no way resembles the panned prequels.
Lucasfilm stopped short of saying the prequel 3D conversions are cancelled, opting instead to state, "we will post further information about our 3D release plans at a later date."
Screenwriter Michael Arndt has been working on the Episode VII script since late last fall. Even though Abrams is now on board, he will have to finish up post-production and media rounds on Star Trek Into Darkness before diving into Star Wars. The summer 2015 release date might be in jeopardy because of this, especially since every past Star Wars film has taken a full three years to put together.