What exactly is Nanoha's "dead" expression?
See this image:



The image consists of comparison shots between the TV broadcast (left) and the DVDs (right). Except for the bottom right one, obviously. The rest are legitimate images.

This is the infamous white devil scene from StrikerS, and you shouldn't have much trouble noticing significant differences in the faces, especially in Nanoha's.

The severity of the difference should also suggest why it was changed: they decided Nanoha's reaction and characterization in that scene were inappropriately grim and harsh. In the broadcast, she is mortified into brutality. In the DVDs, she is disappointed but cool and in control.