Ask HN: How difficult is it to change specializations as a software engineer?

Background: I have a degree in Computer Science from a good University, and have been doing professional web development for about 3 years since graduating. I fee like I am pretty OK at what I do, but I'm not super passionate about it. I would welcome a change to something different (a higher salary would certainly be nice too), but I don't really know how to go around doing it. I really enjoyed doing AI and parallel computing stuff in college, but I can't see taking a course on a subject ~4 years ago meaning much on a job application, and I think I'm too old for an internship level to make sense. If, for example, I go through a machine learning course on Udacity, is that enough for a job in that specialization? There are things that I think I would really enjoy doing like audio processing, but I have no idea how to get into that industry, especially as a 30 year old that would be competing against fresh college grads for a position.

tl;dr: How do I not get "stuck" doing web dev?