I don't see a flip-flop here, his position is how it has been for over a year:
http://apnews.myway.com/article/20040810/D84CJ5MG0.html
In October 2002, Kerry voted to give Bush the military authority to oust Iraqi President Saddam Hussein - a vote he said Monday that he stands by despite the failure to find weapons of mass destruction. But the four-term Massachusetts senator added that Bush used the authority poorly, rushing to war with limited allied support and little thought to an end game.
Kerry's national security adviser, Rand Beers, said Tuesday that "the issue has never been whether we were right to hold Saddam accountable, the issue is that we went to war without our allies, without properly equipping our troops and without a plan to win the peace."