How does this quotation from "Composed upon Westminster Bridge, September 3, 1802" change the tone of the poem?Dear God! the very houses seem...

The poem does change in tone, but the shift occurs earlier
than in the line you cite here. Consider it in
context:

Ne'er
saw I, never felt a calm so deep!
The river glideth at his own sweet
will:
Dear God! the very houses seem asleep;
And all that mighty
heart is lying still!

These
are the final lines in the poem, and for the first time in the poem, the exclamation
point has been employed, indicating a change in tone.

From
the beginning of the poem until these concluding lines, the speaker has described the
beauty and serenity of London as it appears to him in the early morning hours. It is a
city asleep; the air is clean and the city is quiet. He describes it as an extension of
nature itself, as it is "Open unto the fields, and to the sky." The tone is one of quiet
reverence.

In the final four lines, however, the tone
changes. The speaker is no longer merely an observer; he has been drawn into the moment.
The peaceful calm of the city has infused his spirit; he, too, now feels serene. Thus
"Dear God!" expresses his joy and wonder in the experience. His feelings are so all
encompassing and profound that every part of the city, even the houses themselves,
affect him. Every part of the "mighty heart" of London exists in a state of peace,
reflective of his own.

An interesting feature of
Wordsworth's poem its subject. In Romantic literature, spiritual experiences are most
usually inspired by nature. In this poem, London is presented in these circumstances as
being more beautiful and awe-inspiring than nature itself.