That Isha Koppikar's character (Anita) in the new 'Don' was not in the original version happens to be the popular conception, or should we say, misconception in people's minds. There was a talk that Isha's character is a new introduction.
But we hear that Isha's character in the modern 'Don' was indeed present in the original 'Don' as well but went unnoticed because it was a very insignificant one. Director Farhan Akhtar subsequently rewrote and increased it.
When asked, Farhan confirmed the question saying, "There's a character in the original, which is the character of Don's girlfriend. She had a very minor part in the film; in fact she has barely two scenes and is standing on the side in one of the songs looking jealous. But Isha's character in the new version has been substantially developed and when you watch the film, you'll find out yourself!"
We can't wait, as 'Don' releases in the UK on 19th October, a day earlier than the rest of the world.