Well in an ideal world if Jadeja continues to improves his bowling, he good at the moment but really pushes on, then Irfan could replace Harbhajan. That is on the only proviso that Irfan's bowling returns to the 140kph pace with the swing.
Then India would have 4 fast bowling options, Zaheer, Ishant, Irfan and one other. All would offer differnent things. Zaheer the leader of the attack, master of reverse swing. Ishant, pace and bounce, Irfan swing and the other could be a holding bowler like Munaf.
Then Jadeja, Yuvraj, Raina and Sehwag can be the spin bowling options.