Christian Bale and Marky Mark to be trained in boxing by David O. Russell

The shift in casting for this film, which once had Brad Pitt in a role and Darren Aronofsky directing, comes via this in Variety:

Christian Bale and director David O. Russell are poised to get into the ring with Mark Wahlberg on "The Fighter."
Relativity Media has stepped up to fully finance the film. Paramount Pictures, which initiated the project, will now be limited to distributing domestically.
The picture is expected to begin production in July, though Relativity stressed that the principals’ deals are still being negotiated.
Pic tells the story of Boston fighter "Irish" Mickey Ward and how he was helped to the world lightweight championship by half-brother Dicky Eklund. Eklund once decked Sugar Ray Leonard and went the distance against the boxing legend before forfeiting his career to drugs and crime. He redeemed himself by training Ward through his Rocky-like run to the title.
Project reteams Bale with Relativity and its fledgling one-off picture division, which produced the Bale starrer "3:10 to Yuma." Over the past few years, "The Fighter" has drawn some of Hollywood’s biggest talents but was KO’d on two previous occasions. The project first came together with Boston natives Wahlberg and Matt Damon toplining for director Darren Aronofsky. Damon dropped out and Brad Pitt was poised to replace him, but the picture still stalled. Then Aronofsky moved on.
Meanwhile, Wahlberg has continued to train for the pic and is in fighting shape.
David Hoberman and Todd Lieberman are producing through their Mandeville banner. Most recent script drafts were by Scott Silver and Lewis Colick (who rewrote the upcoming Zac Efron starrer "The Death and Life of Charlie St. Cloud").
Wahlberg will play Ward, a fighter who was losing bouts and was ready to hang up the gloves when his brother came back into his life. Bale will play Eklund, whose drugs and robbery spree drew him a 10- to 15-year sentence in state prison. There, he kicked drugs, became a model prisoner and emerged as a changed man who helped his brother reach the glory that eluded him. Russell previously worked with Wahlberg on "Three Kings" and "I Heart Huckabees." The director gets in the ring for the first time with Bale, who’ll next star in the McG-directed "Terminator Salvation" and the Michael Mann-helmed "Public Enemies." The director, who is repped by CAAhas signed on for three projects in recent weeks including "The Fighter": "Aaron and Sarah" at Fox 2000 and "The Silver Linings Playbook" for the Weinstein Co.
-Aronofsky must have not wanted to get boxed in (no pun intended) with sports movies, and also has Robocop on the horizon, so that change was inevitable. I like where this film is going, but it's still early...what do you think?