Okay, so he doesn't say "change" in this line, but the speaker is talking about light as if it's a river—streams of light, so to speak, falling from the sky. So while he doesn't say things are changing, he's using words that bring out the feeling of motion and movement, rather than things that are still. And this could suggest change, right?
All the sun long it was running, it was lovely, the hay Fields high as the house, the tunes from the chimneys, it was air And playing, lovely and watery And fire green as grass. (19-22)
The speaker comes out and says that life on the farm was lovely and beautiful. But, the speaker is also describing the landscape as something in motion. He says "it was running," and the pronoun "it" is open for interpretation. "It" could be youth, a typical day at the farm, the land itself, but also time and the experience of change. Things are in motion and changing all the time. Just like that saying "you can never step into the same river twice," life is constantly changing at the farm for the speaker, whether he accepts that change or not.
Notice, he also uses the word "watery" and "fire." Again, these are simple, beautiful descriptions of his life on the farm that simultaneously hint that things aren't exactly permanent. "Watery" suggests something fluid that can morph into new shapes, just as "fire" is an element that can change whatever it is heating. So while the speaker might just be describing his happy childhood on the farm, he's also subtly suggesting that things can and will change—though he may not have known it back then.
Under the new made clouds and happy as the heart was long, In the sun born over and over (38-39)
Ever sat back in a field and stared up at the clouds? Ever watched as they morphed into different shapes right before your eyes? In a way, the "new made clouds" could allude to this transformative quality. Also, the sun is "born over and over" so it is constantly made new again, lucky feller. Although this could mean that the sun only lives one day and then is recreated anew each day after that, it also suggests a constant state of change as time marches on. The genius of these lines lies in how Thomas used words that suggest two opposite things at once. On one hand, life is new and fresh every day, and it'll never change. On the other hand, life is constant motion, unstable, and changing right before the speaker's eyes.
And nothing I cared, at my sky blue trades, that time allows In all his tuneful turning so few and such morning songs (42-43)
The key here is the word turning. What he thought was an endless childhood has become "so few and such morning songs." Things have changed, and looking back, the speaker realizes now that he's older that his days were limited. Also, he uses the word "morning" but he could also be alluding to "mourning," as in, his poem is a "mourning song" about youth. He misses how things used to be and there's nothing he can do now. Too bad, so sad.
Time held me green and dying Though I sang in my chains like the sea (53-54)
At the end of the poem, the speaker's perspective on youth has shifted in a major way. He's changed now that he's older and he describes himself as "dying." Earlier in the poem, youth on the farm was full of life, but now it's the opposite. He sees it as a time of dying, even though back then, he felt very much alive. It's a sort of a metaphorical death. He wasn't actually, physically dying, but his youth was dying, and what's worse, he didn't realize it until too late. Finally, in the last line of the poem, the speaker puts two opposite things next to each other, possibly to suggest the paradoxical nature of youth seeming both eternal and too brief. "Chains" suggest imprisonment, while the "sea" is fluid and in motion and vast. Normally, we don't think of the sea as chained but as something free to move however it wants. One effect of placing these two words next to each other is to suggest a feeling of being both trapped and changing through time.