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:

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Thus, though we cannot make our
sun

Stand still, yet we will make him
run.