From the article:
Alan Horn, Warner's president and chief operating officer, cited two reasons for the move. "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," Horn said in a statement. "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's motion picture group, added in a statement that the move will not affect production of any future Potter films. (The seventh and final book in J.K. Rowling's series, Harry Potter and the Deathly Hallows, will be adapted as two movies.)