What change is it that you are refering to in this great poem? Do you mean the way in which the final stanza suddenly projects the speaker into his own imagined future when he will look back and think of the choice he made? I will assume this is the case, but please respond to my answer if I have not understood your question correctly.
Let us focus on the final stanza and its content:
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I--
I took the one less travelled by,
And that has made all the difference.
I think one of the major techniques that is used in this final stanza is that of repetition. Note the way in which the third line is basically a copy of the first line of the entire poem. This of course helps to conclude the poem but also leads towards the way in which this is a momentous decision that the speaker has to make. It helps us focus on the way in which this one decision has "made all the difference" in terms of the future of the speaker's life. Choosing the path "less travelled" has resulted in one specific kind of future, whereas, it is suggested, if he had taken the other path, he would have had a different kind of future.
The use of the word "sigh" in this last stanza greatly interests me. It seems to point towards a kind of introspective rumination about the kind of life that the speaker would have experienced had he selected the other path. The title of the poem, focusing on "The Road Not Taken," likewise reinforces this view. To me, this captures the way in which we can often find ourselves haunted by the decisions we have taken in our lives and the way that they have resulted in our present realities. Major life decisions like not marrying or marrying somebody, moving to a different city to take a job or refusing such an opportunity could radically impact our lives, just like the two paths that lie before the speaker. Perhaps the final stanza indicates a change of tone to a more introspective, meditative wondering about different possible futures that the speaker could have enjoyed, but at the same time sadness regarding the way that such alternative futures have been irrevocably lost with the choice of one path over another.