The much-awaited film, directed, co-written and co-produced by Madhur Bhandarkar, will now release on September 21.
An insider told the Hindustan Times : "The clash wasn't planned at all. When it came to our notice that Bebo's birthday was a week away from the original release date, we took a unanimous decision."
Director Madhur Bhandarkar said: "We decided to make things special for her (Kareena) with a bit of a reshuffle."
Kapoor plays Mahi Arora in the film which chronicles the rise and fall of a movie star.
The actress said: "I would call it (Heroine) my best work till date. After Jab We Met I think I have really put my sweat and blood into this film. So what better birthday gift than it coming out on the same day?"
Kapoor is currently shooting for the song 'Halkat Jawani' with choreographer Ganesh Acharya.
The actress described it as 'the raunchiest number' she has ever done.
Heroine's release clashes with two other female-centric films, Sridevi's comeback film English Vinglish and Preity Zinta's Ishkq in Paris, which are set to be released on the same day.