Hardly a shift. Bush gave numerous speeches preceeding any action on the war on terrorism and how that changed the dynamics. You must of missed it. It was carried live and in all the papers. Maybe you were out of the country at the time.
Later, when it became apparent that it was Iraq's turn to come clean the administration listed a number of benefits that would come from going into Iraq. These included liberating Iraq, regime change and WMD.
I know its hard for anti-bushies to hold more than one thought in their heads at one time and all but even us common conservatives can se that Bush presented a logical progression in his argument to remove Saddam by force.