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“CSI” is probably a strong enough concept to survive cast changes, witness the success of the two spinoffs. But until now, its producers and CBS have never had to wonder. The original cast from the pilot episode-Petersen, Helgenberger, George Eads, Gary Dourdan, and Paul Guilfoyle-is still intact. Jorja Fox joined the cast for the second episode, and her Sara Sidle character becomes the first major departure from the show tonight. Sara’s disenchantment with the unending parade of crime has been one of the themes to the season so far, so tonight’s episode figures to be emotional. Her relationship with Petersen’s Grissom, the revelation of which may have been the biggest shocker ever for a show that deals with corpses, was controversial for a subgroup of fans who worried that “Grey’s Anatomymy” might scare the producers into soaping it up, but it has been handled in a way that respects what we’ve learned about both characters over eight seasons.

Leaving “CSI” could not have been an easy decision for Fox, who bounced around TV for years, most notably in a somewhat thankless semi-regular role on “ER”, before striking gold in Las Vegas. She may not be the last to leave. Petersen in particular has made noises about turning in his bugs, so it might behoove fans to keep looking for little signs, such as Grissom’s confession to Anthony LaPaglia’s Malone character at the end of last week’s “Without a Trace” crossover, that “CSI” might soon be in for its biggest change yet.