The answer can be found in Part I, Chapter 4 of 1984:
In the speech under review, Big Brother had referred to an organization called the FFCC, praised it and singled out an official, Withers, for special commendation. Now the organization no longer existed and Withers was an “unperson.” No one usually knew what happened to those declared as “unpersons,” public executions or trials of political offenders were spectacles which happened only once every two years or so. Usually such people just disappeared.
Now Winston had to rewrite Big Brother’s speech without any reference to FFCC or Withers. He decides to invent a totally new person as the subject of the speech and names this imaginary character Comrade Oglivy. Of course, Comrade Oglivy did not exist, but once Big Brother’s speech about him was placed in the newspapers with a couple of faked photographs his existence would become indisputable fact. Winston writes the speech in which Big Brother pays glowing tribute to the heroic life and glorious death of Comrade Oglivy who is held up as an example for all citizens to follow.
The fate of these two foreshadows what will happen to Winston and Julia: by the end of the novel they too will become unpersons. The Party will falsify their past written records so that it appears--on paper--that they never existed.
The "mutability of the past" shows that the Party is a well-oiled machine of censorship and disinformation. Since they control all newspaper and language in the Ministry of Truth, the Outer Party and Proles believe their propaganda. Winston's job, after all, is to erase names and pictures of party dissidents. The Party's ability to change history and language enables them to torture and dis-inform, and it strengthens their control over the masses, making it nearly impossible to organize and rebel.