The first part of this question was asked a few days ago. Please follow the link that I have provided below to see the answer to that part of your question.
As for the second part of the question, I do not think there is any real point to changing opponents. Orwell never really tells us why or how the countries pick who to fight and who to be allies with. I think that Orwell has this happen in the story to make two points.
First, he is saying that the opponent doesn't matter. What's important is that the country should always be at war.
Second, he's showing us how much control the Party has over the people -- the Party can convince people that they have always been at war with some country even if they were friends with that country just the day before.