Anything they want to do. The majority flip sides and do defense work and don't change fields because they only want to do criminal law. But I worked with an ADA that became a clerk for a Federal Judge and then got a job in a civil law firm. He is now back in prosecution in the Federal system because he hated the civil work.
And you can usually make the same or better money by doing defense work, but the two biggest reasons for the flip are simple. 1. They only did prosecution to learn the ropes before they went out to hang their shingle. They weren't really wanting to be prosecutors. and 2. when you hang your own shingle, you become your own boss. You work how you want to, when you want (with the exception of court appearances that are set by the court) and handle your office the way you want. No more following someone else's rules and punching a time clock.
I plan to be an ADA and can't see myself ever flipping sides. But, if I did, I'd do it for the 2nd reason. Being your own boss does sound appealing.