What is the change in tone after line 20 in "To His Coy Mistress"?
You have correctly identified a key turning point in the poem. Remember what this poem is all about: the speaker is trying to persuade his audience to love him now and not to be "coy." The speaker dwells on the details of human mortality with morbid precision, to make his beloved feel that even immoral behaviour while alive is preferable to being good but dead. The first section of the poem slowly and languidly talks about how he would court his love if he had time:
We would sit down, and think which way
To walk, and pass our long love's day.
The speaker says he would spend a very long time praising her beauty and her various qualities, because she "deserves this state" and he would certainly never want to "love at lower rate." So the poem in this part is slow and sure in its praise of the beauty of the love of the speaker. However, it is in line 21 that we see a distinct change of tone - from relaxed to rushed and hurried. The voice of the speaker becomes urgent as he hears "Time's winged chariot hurrying near" and he reflects upon the "Deserts of vast eternity" that lie before them because of the mortality of human beings. Thus the message emerging from this change of tone is clear - we do not have all eternity, because we are going to die soon, so seize the day and love me with such passion and intensity:
Thus, though we cannot make our sun
Stand still, yet we will make him run.