There are many ways in which Big Brother accomplishes this "change." Part of it is through a constant and consistent withering down of individual resistance so that the individual has little choice but to end up accepting the totality of Big Brother. This is seen in Big Brother's use of technology, which allows it access to individual thoughts. It is also seen in the lack of a private realm, indicating that individuals cannot retreat \ else as they can only exist in the public realm. When Winston is caught, it is not merely enough that he says he won't see Julia. He has to renounce and betray her. This is another example of how Big Brother does not destroy, but seeks to "change" an adversary. Through the use of fear and manipulation of individual weakness, one see again this idea of "changing" an adversary.
Think about what is more useful.
Fight and kill an enemy or just let him become an ally that shares and believes in the idea or ideal that you are going to promote as "the best" of the possible worlds?
The enemy of the Big Brother is the individual thought. If every book is rewritten and history has changed, thoughts are rewritten too.
If every newspaper or broadcast on tv is saying just the same thing and discuss an argument in the same way, nobody would have another opinion, so there is no enemy to fear about.
Fear against fear is one of the most powerful way to control our opinions.
Julia is betrayed by Winston's most terrifying fear.