One good example of symbolism is the status of "firemen" themselves. In earlier times, before houses were perfectly fireproofed, firemen responded to fires and put them out; now, they carry flamethrowers and start fires, making them feared across the country:
...she said, thoughtfully, "You know, I'm not afraid of you at all."
He was surprised. "Why should you be?"
"So many people are. Afraid of firemen, I mean. But you're just a man, after all..."
(Bradbury, Fahrenheit 451, Google Books)
The symbol of the fireman has been changed from relieving to terrifying; all the equipment has been altered, from the firetrucks with their tanks of kerosene to the Mechanical Hound, an evil parody of the firehouse dalmatian. With these changes, "fireman" takes on a different meaning, and so the story is changed in turn; were Montag a conventional fireman, there would be no conflict and so no story.