Overlay Tile Restrictions

Based on the rulebook context:

• Overlay tiles are placed on top of map tiles to provide additional features for the scenario (p. 13).
• The type and placement of overlay tiles are defined by the scenario layout and scenario key, not by player actions during play (p. 13).
• Overlay tiles are not tokens, and their placement is determined at scenario setup or by scenario-specific rules (p. 13).
• There are no general rules in the provided context for moving or manipulating overlay tiles during play.

Moving Overlay Tiles

• The rulebook does not describe any mechanism for moving overlay tiles once they are placed.
• Overlay tiles are static features unless a scenario's special rules state otherwise.

Overlay Tiles and Occupied Hexes

• Overlay tiles are placed during setup, and the context does not mention placing them into hexes already occupied by figures or other overlay tiles (p. 13).
• A hex with no overlay tiles (except corridors and pressure plates) is considered featureless (p. 13).
• The context does not provide rules for moving overlay tiles into occupied hexes, nor does it suggest that overlay tiles can be moved at all.

Summary

• Overlay tiles cannot be moved or manipulated during play unless a scenario's special rules explicitly allow it.
• There are no rules for moving overlay tiles into occupied hexes.
• Overlay tiles are static features defined by scenario setup and special rules.

Source: p. 13

If you need to know about a scenario-specific effect that allows overlay tile movement, you would need to consult that scenario's special rules.

Source: Core Rulebook (pp. 11, 13, 14, 15, 75)