What is the word that signals a shift in the poem?

In Shakespeare's Sonnet 18, it is the word "But" at the start of line 9 that signals a shift in the poem.

Prior to that word, Shakespeare sets up the idea that beautiful things ultimately lose their beauty, and that a moment of perfection is quickly erased.  On a lovely summer day, for example, sometimes the sun gets too hot or goes behind a cloud.

"But," Shakespeare argues, the subject of his poem will neither lose (her?) beauty nor fade away.  In writing about her in her moment of perfection, Shakespeare will immortalize her for all time.  In the final couplet, Shakespeare concludes that as long as there are people on the earth who are able to read, the subject of his poem will live forever in her present beautiful state.

Thus, the "but" in line 9 serves as the transition from the concept of a beauty that fades to one that, through literature, lives forever.