Why did Al Pacino drastically change his voice from clean to rasping?
As far as I know, an actor voice is something that does not change much, but I realized that Al Pacino's voice has changed a lot in a short amount of time.
In The Godfather, part II (1974) he has a clean voice (extract here).
20 years is not a short amount of time.
Regardless, Pacino's voice has changed over the years, which is natural. In addition, his acting style has changed signficantly from the early Godfather days to more "shouty" characters...at least that is what he is known for.
However, he has been a smoker for many years.
Stopped a two-pack-a-day smoking habit to protect his voice (1994). In the mid-1980s, he had been smoking four packs of cigarettes a day. He now only occasionally smokes herbal cigarettes.
It seems clear that Pacino was well aware that his voice was under threat and took steps to amend his lifestyle (at least moderately) to protect it.