What does Big Brother represent? Does this change over the course of the novel?
Big Brother is the physical representation of the oppressive government of Oceana. It's not clear whether or not Big Brother is an actual person, but I do not believe that he is. There are posters of Big Brother all over the place, but I believe that he is a representation of the government (as your question implies). "Big Brother is Watching You" is what is written on the posters, and there is no way one guy could watch everybody. But a government body could. Orwell does a nice job of explaining in chapter 1 how a government could do that.
"The telescreen received and transmitted simultaneously. Any sound that Winston made, above the level of a very low whisper, would be picked up by it, moreover, so long as he remained within the field of vision which the metal plaque commanded, he could be seen as well as heard. There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time. But at any rate they could plug in your wire whenever they wanted to. You had to live — did live, from habit that became instinct — in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized."
In a nutshell, Big Brother represents government control and oppression. It watches everything citizens do. It controls their daily schedule. It controls their thoughts. It attempts to control their emotions. It even controls the past.
’In memory. Very well, then. We, the Party, control all records, and we control all memories. Then we control the past, do we not?’
I do not believe its meanings changed over the course of the novel. I believe its powerful grasp was simply driven further into the reader's mind and Winston's mind.