I was searching for updates about the movie and I found some official statements from Warner Bros on why they move the release of the movie next year...
Alan Horn, President and Chief Operating Officer, Warner Bros. stated:
Our reasons for shifting Half-Blood Prince to summer are twofold: we know the summer season is an ideal window for a family tent pole release, as proven by the success of our last Harry Potter film, which is the second-highest grossing film in the franchise, behind only the first installment. Additionally, like every other studio, we are still feeling the repercussions of the writers’ strike, which impacted the readiness of scripts for other films—changing the competitive landscape for 2009 and offering new windows of opportunity that we wanted to take advantage of. We agreed the best strategy was to move Half-Blood Prince to July, where it perfectly fills the gap for a major tent pole release for mid-summer.
Jeff Robinov, President of Warner Bros. Motion Picture Group, confirmed:
The release date change does not alter the production schedule for this or future Harry Potter films. Post-production on Half-Blood Prince was completed on time, and the studio’s release plans for the two-part Harry Potter and the Deathly Hallows will not be affected by this change. We know Harry Potter fans are eagerly anticipating seeing the final chapters unfold onscreen. In fact, the good news for them is that the gap will now be shortened between Half-Blood Prince and the first part of Harry Potter and the Deathly Hallows.
While producer David Heyman offered:
When Jeff Robinov explained the rationale behind moving the release date of Harry Potter and the Half-Blood Prince to July 2009, it was immediately apparent that this offered us the potential to reach the widest possible audience. I am extremely proud of this latest film and of the work of David Yates and our incomparable cast; I believe we have developed and pushed the series further still. We are all looking forward to sharing it with Harry Potter fans around the world, even if we have to wait just a bit longer.