Rachelle Lefevre got herself replaced by Bryce Dallas Howard for the third Twilight movie. What could have possibly transpired to have caused her dismissal?

For her part, Lefevre puts he blame on an uncooperative studio not allowing her to work on another movie:

I was fully committed to the Twilight saga, and to the portrayal of Victoria. I turned down several other film opportunities and, in accordance with my contractual rights, accepted only roles that would involve very short shooting schedules. My commitment to [Canadian indie movie] Barney's Version is only 10 days. Summit picked up my option for Eclipse. Although the production schedule for Eclipse is over three months long, Summit said they had a conflict during those 10 days and would not accommodate me. Given the length of filming for Eclipse, never did I fathom I would lose the role over a 10-day overlap. I was happy with my contract with Summit and was fully prepared to continue to honor it. Summit chose simply to recast the part.

Summit immediately put out a press release that was pretty much "Everything she said? Forget it.":

We at Summit Entertainment are disappointed by Rachelle Lefevre's recent comments which attempt to make her career choices the fault of the Studio. Her decision to discuss her version of the scheduling challenges publicly has forced the Studio to set the record straight and correct the facts... If Ms. Lefevre was, as she describes "passionate," about being part of THE TWILIGHT SAGA, we feel that she and her representatives would have included us in her decision to work on another film that would conflict with the shooting schedule of THE TWILIGHT SAGA: ECLIPSE.

All this studio actor beef could just turn out to be more interesting than the show itself. Granted if you're going to watch it in the first place. 

Rachelle Lefevre: Wronged or uncooperative? [Variety]