The inner party is affected the least; they are the ones that actually implement and dictate the changes that are supposed to occur. They decide, based on resources, the mood of the public, and their own feeling of control, whether or not a history change is due. If resources dictate a change, they make it, and make the changes thorough, in order to control the populace. They have to maintain their reputations as trustworthy sources of truth; otherwise, the people might realize that they are frauds and rebel. The changing of history solidifies their control.
For the Outer Party, the impact is varied. One thing that can be said for all of them is that the changes in history keep them busy. Think of Winston's job--it is entirely composed of changing history. It keeps him busy, keeps him involved in a task, and the doing of that task and the staying busy itself is a form of control; at the end of the day, they are so tired that they can't do anything else. So it's a way for the Inner Party to keep the Outer Party tired, and subservient. The changing of history also makes some people mad--like Winston. He realizes the falseness of it all, and is baffled and frustrated by it. Others, however, become completely engrossed in the work of it, to the point of obsession. Changes give them one more mind challenge to tackle, in the name of the Party.
For the proles, changing history just keeps them supplied with a constant and ever-changing source for their frustrations, and fuel to their patriotism. If the proles have a common enemy, then they won't get mad at the party. If they can release their frustrations with life on the evil Eurasians, for example, then they won't rise up; they won't have the energy or inclination to rebel elsewhere. It gives the proles a fresh outlet for angst.
I hope that those thoughts helepd; good luck!