Claire Danes has replaced Sophia Bush in a pivotal role in “Terminator 3: Rise of the Machines,” the Jonathan Mostow-directed pic that stars Arnold Schwarzenegger, Nick Stahl and newcomer Kristanna Loken as the latest model cyborg villain. Filming on the 100-day shoot began April 15.

Danes started work Sunday, taking over as the romantic interest of John Conner (Stahl), who is now in his 20s and once again partnered with his muscular cyborg pal while trying to elude a more advanced TX model robot bent on terminating him to change the future. Mostow felt that newcomer Bush seemed a bit young for the part. Danes, whose deal was quickly made by UTA and manager Michael Aglion, is 23.

“This was a difficult decision because Sophia is a talented and charismatic performer,” said Mostow. “However, we feel, after reviewing the dailies, that her appearance and manner play younger than what we wanted for the role. We look forward to working with Sophia on future projects.”

It is a tough break for a young actress, but the pressure on Mostow and producers Mario Kassar, Andy Vajna, Joel Michaels, Hal Lieberman and Colin Wilson is enormous. Mostow is filling big shoes following hallowed groundwork laid by James Cameron. The Intermedia Films-financed drama’s $155 million-$170 million budget was the highest ever at greenlight stage, with Schwarzenegger pulling down a $30 million payday. The film is expected to be a big summer 2003 domestic entry for Warner Bros.; Sony will handle most foreign territories.

Danes, best known for starring with Leonardo DiCaprio in the Baz Luhrmann-directed “Romeo + Juliet,” was able to step in because she has taken a year off between her junior and senior years at Yale to log a few films. This is the fourth she has matriculated in that time. She stars with Julianne Moore, Nicole Kidman and Meryl Streep in the Stephen Daldry-directed “The Hours” at Paramount. just wrapped starring with Joaquin Phoenix in the indie “It’s All About Love” and wrapped starring opposite Kieran Culkin and Susan Sarandon in MGM’s “Igby Goes Down.”