In reading 1984 we very quickly learn that the Party subjugated the people not as a means to a goal, but instead that this subjugation was the goal.
In the same manner, the Party did not suddenly change the Official Truth in order to protect its grip on power, but instead did this to validate that it still held that power. They wanted to see if the people's capacity to think independently was still crippled to the point that they would forget, on demand, everything that had been shouted to them in the months or even years leading up to that point.
Its has been observed that <PERSON> (and, therefore, we) did not truly know whether any of the conditions outside of his direct experience were as he had been led to believe. For all he (and we) knew, Airstrip One could have been the only dictatorship on the planet, with the remainder of the world watching it in horrid disappointment.
In the same manner, while the Party may want to believe that it has the proles and Outer Party members habituated to uncritically accept what they are told to believe, it knows in its heart of hearts that it can never be 100% certain that any of the people really believes the propaganda. 865
For all it knows, every single prole and Outer Party member could be going through the motions, waiting for the moment when it is safe to rebel.
The Party also officially claims to be the best thing that ever happened to the underclass, when in truth it knows, but can never admit, that the underclass can best serve its own interests by overthrowing the Party. (This is why a dictator is often so convinced that there are traitors among his people; overthrowing him is the smartest thing his people could do.)
These two facts combine to put the Party in chronic fear of overthrow.
Changing the official Party line from time to time is one way the Party deals with this fear. By switching the official story, the Party hopes to expose people who are only going through the motions. If everyone goes along, then things are going as they were; but if large groups of people call shenanigans, then the Party knows that its grip on power is weakening. 999