If you've ever paid cash for something, you know that change can mean a lot of things. In this case, when we say change, we're really talking about transience. What does that mean? Transience is "passing with time" or "the state of being brief or short-lived." In short, we mean a really quick, don't-blink-or-you'll-miss-it change. "Route of Evanescence" is about a brief event, and how lucky the speaker was to catch it. In fact, one reason the speaker seems so wowed by the hummingbird is because of how quick the bird moves in and out of view. If the speaker had looked away for just a second, she may not have seen the bird. The poem is almost suggesting that we pay attention in order not to miss out on things like this.
Whoosh! Zip! The poem tries to imitate the hummingbird by describing events quickly.
If the event being witnessed by the speaker had not happened so suddenly, it would not have had a lasting effect. Ironic much?