While that scheduling upset Potter fans, the decision to split the final book in the series into two films got a warmer reception from devotees happy to squeeze every last drop of magic potion from the story of the boy wizard.
J.K. Rowling published her final Potter tome, "Harry Potter and the Deathly Hallows", last year but the film franchise, which has already earned 4.5 billion dollars (3.2 billion euros), will stretch into 2011.
"Harry Potter and the Half-Blood Prince" opens in most of Europe on July 15, and in the United States and Japan two days later.
It reunites the film's three young stars, Daniel Radcliffe as Harry, Emma Watson as Hermione Granger and Rupert Grint as Ron Weasley, but their fresh-faced childhood innocence is a distant memory.
A growing evil force hangs over Hogwarts. The school's one-time potions professor Horace Slughorn, played by Jim Broadbent, an Oscar winner in "Iris" and a new addition to the cast, is lured back to his former post to find the key to breaking down the evil Lord Voldemort's defences.
But the students' focus on the looming final battle between good and evil is obscured by their growing attraction to each other.
Harry had his first kiss in the fifth film of the series, "Harry Potter and the Order of the Phoenix", but now his relationship with Ron's sister, Ginny, is becoming something more serious.
Meanwhile the coquettish Lavender Brown (played by Jessie Cave) fixes her sights on the gawky Ron who is oblivious to the fact that Hermione has been holding a torch for him.
"For me, the films have always been about a loss of innocence," said Radcliffe. "When Harry came into this world, it was all just amazing and brilliant and kind of pure. But as the films have gone on, that's totally disintegrated, and he's realising that the wizarding world has just as many, if not more, challenges than the world he grew up in before."
Just as their characters are growing up fast in the film, so Radcliffe, Watson and Grint are charging into adulthood away from the set.
They were aged 11, 10 and 12 respectively when they were chosen for the first Potter film, which hit cinema screens in 2001.
Radcliffe, who turns 20 on July 23, famously appeared naked on stage in New York and London productions of "Equus" to blow away his straitlaced image.
Watson, now 19, radiates understated English chic modelling for fashion house Burberry, her way of blowing away the frumpy jumpers and backcombed hair she wears on set as the academically brilliant Hermione.
"When I was little, I didn't understand that other kids thought I actually was Hermione, really geeky. It was devastating. I thought no one would ever fancy me," Watson said in a recent interview with The Times.
Grint, meanwhile, has had to contend with a mild bout of swine flu but is expected to be fit to attend the premiere.
Stuart Kemp, London correspondent of Hollywood Reporter magazine, said the Potter films had become one of the most successful film franchises in history by evolving with the books' core audience.
"They are getting more grown-up and sinister as the fans grow up with them, and they have become progressively darker.
"When the series does end, it will be a huge loss to the British film industry, both in terms of box office and jobs. They have employed a lot of people and they will leave a sizeable hole."
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