Editing Ogre:CharacterCodex:Combat (section)

===Target===
A target is an opponent on which you have declare an attack that is not resolved yet. Attacks of opportunity are more easy to make on a target than on another opponent in a threaten square. (See Attack of Opportunity below)

Changing target: If for some reason you need to change target while attacking during a fight, you can do it, but incur a -4 penalty to your action. It also take at least one segment to change a target, this mean that you cannot change target on the segment you resolve your attack.