For the section we've been calling "Unofficial Part 1" of the story, we deal with a very opinionated, quite charming, first person narrator, who actually re-emerges right at the end of the story (in the final paragraph). We only hear his thoughts, and his distinctively humorous voice colors our view of the first part of the story.
In the second half, however, this first person narrator abdicates his power, and switches suddenly to the third person. This shift creates something of a sense of alienation and distance – we were used to knowing everything our narrator was thinking, and feeling like we were having a conversation with him, but all of a sudden, we're kind of out in the cold. Through the limited omniscient gaze of the third person narrator, we see the thoughts and feelings of Sergeant X (the character of our first person narrator, seen from a slight distance), but we're somewhat removed from him now. This serves the function of showing us the impact the war has had on Sergeant X over the past year – he's a very different man from the one we met in England.