I sort of arbitrarily chose AI and Platforms as my concentrations for my bachelor's, but I definitely have interests in media. How difficult is it to move laterally in the CS industry - in my case from something like platforms to something like game design?
Impossible without further schooling? Relatively easy? Something else?
If you're willing to continue learning and aren't afraid of having to prove your abilities in your new field, it's not difficult.
It's considerably more difficult if you have considerable experience in one field, and expect to maintain your compensation level in a different field.
Smart, educated people can get a job doing anything anywhere.
I know this must be a fact because also the opposite is true^^
If you are interested in game design, and you can find something to do, ie a position to apply for you will most certainly be evaluated if you can get a interview.
Considering also sometimes being a good colleague is just as important as skills, maybe sometimes even more important than skills.
If you can be smart, persistant and get to talk with the people offering thejob you will be certainly be evaluated. I have many friends and colleagues who majored outside IT, IS that are working inside this domain.
So, my answer to you is you gotta try to find out :)
As always it depends on your skills, the better you are, no matter at what field of CS the easier it will be for you to move from one field to another.
And learning about the field you interested in before hand, wont hurt it for sure.
Be good, be smart, learn, and all doors will open for you.
The tech industry is largely about experience. Sometimes you will work on fringe projects at work that aren't directly related to what you specialize in, but that's still experience. It's a seed. And then it snowballs.
Like if you're a language developer and they ask you to be a DBA in a pinch. You learn some SQL. It's bad, it's ugly, and all you had was Google. But now you "know" SQL. Now you're not just a junior programmer, you're a seasoned programmer with experience in multiple layers.
If work doesn't offer you this opportunity for fringe experience, there's always hobby/out-of-work/open-source stuff. You can make up for the lack of formal training and formal experience by giving yourself work. It won't be easy, but the option is always there.
If you're dedicated to your craft, have a genuine desire to do something, and have the brains to back it up, there's really nothing holding you back from jumping around to any arena of tech. Again, it won't be easy, but it's absolutely possible.
Edit: What I originally wrote is still great across the board, except for maybe the gaming industry. I hear they're insular and totally different from everyone else. So the barrier to break in might be much, much higher than say web development to database management. Still doable, but much harder.