The candidate isn't going to complain if you give them _more_.

Why would you sue for that? There's no contract in place

And in any normal court they will lose since its a different position and they agreed (paper trail) for that switch. Or they could be just let go if company is too careful. Not that such situation happens often in real world, I never even heard about even one.

If they would sue it would also burn their professional creds permanently, nobody sane would ever want to hire such a problematic person.