how would they change brain chemistry?
By changing how they think/feel. Thoughts and emotions affect our brain chemistry, everything does. Nobody has a static brain there are numerous processing going on from moment to moment.
And I suppose AA does change thought patterns. But to me it is changing one incorrect belief with another, for example:
"I need to drink to feel good and can't live without it"
changes to..
"I am an alcoholic/addict for the rest of my life and cannot consume any mind-altering substance again, except caffeine or nicotine which do not follow the law of addiction like other substances"
So this new way of thinking can stop somebody from consuming alcohol. But it is based on the idea that there is some inherent insurmountable difference between that person and other people who are able to drink "normally". So somebody may be better off than before they stopped drinking, but it does not mean it's true.
Some people may react more strongly to alcohol or enjoy the effects more, but there is nobody whose body has an actual physiological need to consume alcohol once they have one drink. They may want another drink more than anything else in the world, but their body does not need it to survive.
If somebody is conditioned to always follow through on their impulses, they may feel they have no choice but to continue to drink. But think of it like an emotion...if somebody has a tendency to experience rage then they will have to learn ways to avoid expressing it in ways that are harmful to other people. An alcohol buzz is just like an emotional state, and people can learn to respond to it differently.
It doesn't mean it's easy or even desirable to do so. It may be simpler just to not drink at all. The only reason I take issue with this is because many people are told that it is an impossibility which then necessitates the forever-an-addict identity, which I feel does more harm than good for some people.