Here's what Kerry said on the Senate floor before voting to give Bush the authority (to use force on Iraq):
Kerry (Oct. 9, 2002) Let there be no doubt or confusion about where we stand on this. I will support a multilateral effort to disarm him (Saddam) by force, if we ever exhaust those other options, as the President has promised, but I will not support a unilateral U.S. war against Iraq unless that threat is imminent and the multilateral effort has not proven possible under any circumstances.
Change in his position? There's been no change.