I could possibly see them phase the old characters out and bring in new ones. Like what they had going in season 9: the new CSIs (Riley and Ray) under leadership of the old (Cath, Nick and Greg). When one actor leaves, they could bring in a new one. It would make the transition easier for many to swallow in oppose to a whole new cast just out of the blue.
When Cath does leave, tptb will probably bring in someone new - that is if they don't just cancel the show at the end of the season. As time goes by, the other older characters could be written out. Sara goes back to Grissom, Greg transfers somewhere else, Hodges chases after Wendy, Robbins retires, Nick gets bitten by a spider he's playing with and dies...
When Cath does leave, tptb will probably bring in someone new - that is if they don't just cancel the show at the end of the season. As time goes by, the other older characters could be written out. Sara goes back to Grissom, Greg transfers somewhere else, Hodges chases after Wendy, Robbins retires, Nick gets bitten by a spider he's playing with and dies...