In my opinion, the move from the Romantic to the Modernist comes starkly in the last stanza. In reality, before this point, there is talk about "snow" and "harness bells" and "sweep of easy wind" and "downy flake": images very romantic in nature! Not in the final stanza, however:
The woods are lovely, dark, and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
Whether the worries of the speaker are with the rat-race of society or with death, there is negativity here. The entire stanza, specifically the very repetition of the final line, rejects the positive and relaxing images of the peaceful winter scene. The final stanza is obviously meant to put a disturbing twist on winter beauty.