The future perfect tense indicates completion in the future:
By tomorrow afternoon, he will have changed his route.
When I'm 65, I will have saved up a fortune.
In two more weeks, you will have seen every car in the museum.
However, this tense works like the present perfect: it puts the verb into a whole time interval ending at some time in the future. It's suitable for accumulations (like saving up money), repeated events, and actions completed at an unspecified time before a specified future time—hence the by in the first example. It doesn't make sense if you say a specific time in the future for the event:
At 3:00 p.m. tomorrow, he will have changed his route.
If you mean that the act of changing his route will occur at precisely 3:00 p.m., then you use the simple future tense:
At 3:00 p.m. tomorrow, he will change his route.
The future present
This sentence describes the future but grammatically it's in the present tense:
Later we realize that he changed his route.
This sentence means that the changing of the route was completed before you realized it, but it doesn't say whether the changing of the route happened before now or after now. The adverb Later with the present tense moves the present tense into the future.* From that future perspective, the changing of the route happened in the past, so it gets the simple past tense. The same is true if you put the main verb into the future tense:
Later we will come to know that they spent a whole day at the computer fair.
What we learn later might be that they spent the day at the computer fair yesterday, today, or next week; the sentence doesn't specify.
English tenses concisely provide a lot of information about the times when actions happen in relation to a reference point in time (like the present, or a specified time in the past or the future), but they're not so precise about absolute time. For that, you need to use specific words, like "tomorrow at 3:00 p.m.", rather than grammar alone.
* Later can even move the present tense into the past—a later point in the past than was just discussed. "Now let's get our story straight before we talk to the police. Two years ago, we think he took the 405 into Los Angeles. Later—that is, last year—we realize that he changed his route."