If all positions on the battlefield are already filled, the player must use Replacing to deploy a new Unit. The rulebook states:

• "If you want to deploy a Unit on the same position as another card of yours, first you must destroy the card already in play that occupies that position, then you may put in play the new Unit. This is called Replacing."
  (Page 15, Section 4.4 - Replacing)

• "Deploying means to assign a Unit, which enters into play, due to a Summoning or by resolving the effect of a card, to an available position on the battlefield. The assigned position must be valid (e.g.: An Acolyte with the Fierce attribute cannot be deployed on a defence position)."
  (Page 14, Section 4.2 - Deploying Units)

This means:

• You cannot deploy a new Unit to a position that is already occupied unless you first destroy the existing card in that position using the Replacing process.
• Deploying restrictions (such as those from attributes like Fierce) still apply when choosing a position.

Summary:
If all positions are filled, you must use Replacing to deploy a new Unit. You cannot simply deploy to an occupied position without first destroying the card there.

Source:
Page 14 (4.2 - Deploying Units), Page 15 (4.4 - Replacing)

Source: Core Rulebook (pp. 14, 15, 24, 31, 33)