A reporter who covers the police beat at a newspaper changes careers. she becomes an editor of nonfiction books and is paid the same salary as she was at the newspaper. what has she experienced?

She has experienced horizontal social mobility.

It means that she has only moved horizontally - her paycheck has stayed the same, she didn't get any special promotions or anything. The only thing that changed is the place where she works - if she were given more salary, it would be a vertical social mobility.