Skinning over main Cliff Tile, Tiles?

I'm trying to change a cityscape dirt (City_Dirt.blp) skin into an ashenvale grass (Ashen_Grass.blp) by changing the skin path so that I can have essentially, two different cliff-types on the same map. I have already re-skinned the cliff types for the ashenvale, so that it overwrites the Cityscape dirt cliff, but theres an edge of cityscape dirt around the cliff, so I need to re-skin the normal City_Dirt.blp to look like Ashenvale grass. Anyways, I made the changes, and imported it as: "TerrainArt\Cityscape\City_Dirt.blp" and it doesn't work, whats the reason for this?