In 1984, why must the past be altered?
According to the principles of ingsoc, the past is completely arbitrary and exists only in the written or other records of a society. Because this is the only reality in which they exist, once those records are destroyed, the past ceases to exist in any form except for that which the ruling party chooses.
And so the ruling party can then change the past to be whatever it wants without ever having changed the past because it only exists in the way the party chooses. Once this mechanical process is mastered by the party, it follows that members of the party and of the society in general must be trained to forget the past as well and be able to subscribe to the manufactured past easily and quickly.