Transformations are a major way Williams approaches nature in this poem. What he describes is "just" the changing of the seasons, but he makes it into a really big event. The world is changing, transforming itself from brown to green, from dead to living, from cold and windy to calm and warm. As Williams puts it: "the profound change has come upon them." It’s the big shift in the poem, so it’s definitely worth underlining here.
By the end of this poem, spring has changed everything, including the speaker.