HD Edition has different terrain textures than 1.0c. They're now pngs that are transformed during render to fit the terrain:

Textures are found in <Age2Dir>/resources/_common/terrain/textures.

Older HD Edition versions also still shipped with 1.0c's terrain SLP files, but they were removed as of patch 4.8. Currently, users have to downgrade to patch 4.3 before running the conversion.

I suppose to convert these new textures, they need to be cut up and transformed to generate the same output as the SLP conversion.

Interesting data:

• The texture names seem to be derived from the name1 attribute on Terrains. eg. the g_wtr texture name is g_wtr_00_color.png.
• The SLP id of the obsolete textures is still present on Terrain data from HD 4.8. So that could be used to map the texture name to an appropriate output file.