In "The Road Not Taken," where is the shift in the poem?

There is a definite shift in terms of time and when the poem is
being narrated in the break between the third and final stanza. The narrator has just made his
decision to follow one path, consoling himself with the thought that he could keep the other path
"for another day." However, as he goes on his chosen path, he reflects that he probably would be
unable to come back and take the other path later:

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Yet knowing how way leads on to
way,

I doubted if I should ever come
back.

The last stanza signifies a
break in the poem as the speaker imagines himself talking about this seemingly harmless incident
later on in life and expressing his sense of curiosity and regret about choosing the path that he
did:

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.

This
shift in the last stanza seems to be related to what Frost is trying to say through the poem.
Every choice is a risk and even carefully weighed choices have unexpected outcomes. The reference
to the "sigh" that the speaker tells his story with helps establish a tone of regret. Although
both paths looked the same, and often in life choices may be very similar, one can turn out to
have radically different outcomes, and we are left wondering what would have happened if we had
taken the "road less travelled."