A charging unit cannot turn, change formation or reform - it may only wheel. This is because the troops are running or galloping once the charge has begun and are unable to execute delicate manoeuvres even if they were able to hear the orders of their leaders above the din.
When a unit charges an enemy, the player must endeavour to bring the maximum number of models from both sides into combat. This can sometimes be achieved by moving the chargers straight forward, but often it will be necessary to wheel slightly to face the enemy.