Enevhar Aldarion
Hero
That's not accurate. Nothing in the rules say you must declare all actions and then blindly follow through with them.
Anyone can change their mind after they see the result of any attack (or any action they do). Even Ready an Action allows you to not take your action after the trigger.
You can move between attacks, so if you drop one foe, you can move to the next and keep attacking.
edit:grammar is hard
And that is why I said it depends on how you run declaring actions. In my games there are no "take backs", so if you say I am using all my attacks on an enemy, then guess what, you are using all your attacks against that enemy, unless you have a feat or ability that specifically lets you retarget after the enemy goes down.