I don't know based on the rulebook. The rulebook context defines the FIERCE attribute as: "This Unit can only be deployed on an attack position" (page 21). It also states that when moving Units, "Deploying restrictions still apply (e.g.: Fierce attribute)" (page 15).

However, the context does not specify what happens if a FIERCE Unit is moved to a defense position by an ability or effect. It is unclear whether the move is prohibited, the Unit is destroyed, or it is returned to its original position. The rulebook only clarifies that FIERCE restricts deployment, not movement, and does not provide a consequence for violating this restriction via movement.

To answer this question, the rulebook would need to specify the outcome when a FIERCE Unit is moved to a non-attack position.

Source: Core Rulebook (pp. 15, 21, 24, 25, 26)