I am inclined to believe that this not a change of heart, but rather political realities forcing her to do the right thing:

Today, EFF joined a broad coalition of other public interest groups at Democratic Leader Nancy Pelosi’s office in San Francisco, to present her with a petition carrying an incredible 209,419 signatures with a request to oppose the introduction of the Trans-Pacific Partnership (TPP) during the post-election “lame duck” session of Congress. And with your help, we succeeded! In a letter that she handed us at our meeting, Leader Pelosi wrote:

As Congress and the American people review the finalized terms of the Trans Pacific Partnership (TPP), we must put American workers first to allow our economy to grow and America to succeed. Please be assured that I will oppose the TPP as it is currently written or any deal that attempts to separate commerce from the environment and will work to ensure that our nation’s trade policies include increased transparency, more consultation, and stronger protections to create jobs, strengthen human rights, and preserve the environment.

Yes, I am rather cynical about this.