Im not sure but what i've heard is that after you quit smoking you can see gradual improvements over the next 1-2 years as your vocal chords are able to move better and hence get stronger with the exercise.

Obviously the hope is that once it improves far enough he can go back to his more natural singing technique which he was using around 1994. What lots of people say is that for last decade he has been using a different singing technique to cope with the diminished voice.