I read the following:
After graduating from Junior High, Kevin and Winnie both go to McKinley High and Paul attends a prep school. Paul would later transfer to McKinley High and join Kevin and Winnie.
The story is narrated in simple present. Why do they use 'would' here?
This is a rather unusual use of would, but a legitimate one nevertheless. It is used here to describe ‘a past event as seen in the future from a point further in the past’. That description is from ‘An A-Z of English Grammar and Usage’ by Leech and others. The example given in the book is:
The building of the bridge was an important event which would be remembered for many years to come.
You're right: this is unusual. The historic present, which is already questionable in such a bland, not very vivid story, should not be accompanied by would but by will, if by any form of will at all; the switch from present to past here is awkward.
In fact, I cannot think of a situation where the historic present goes well with words referring to the future, because they suck the reader out of the "present" perspective. The historic present is supposed to make us feel the scene is actual and real; switching to the future puts it at a distance, since it entails that we can oversee the future.
The quoted text isn't from story's narration, but from the plot synopsis The Wonder Years at Wikipedia, so the tense of the narration is irrelevant.
... After graduating from Junior High, Kevin and Winnie both go to McKinley High and Paul attends a prep school. Paul would later transfer to McKinley High and join Kevin and Winnie.
The synopsis uses would here because it's describing some current events in the story (Kevin and Winnie going to McKinley High), and mentions what Paul will later do on in the story. Right now, he isn't at McKinley High.
This is narration from the Wonder Years, isn't it? The entire series technically occurs in the past and our viewpoint is that of adult Kevin, so it's never really in the present tense for the viewer.
