I've been told that you can no longer substitute cliff sets into a map, but you *can* import skins into the map to replace the cliff set's tile. You actually have to import two skins, one for the cliff itself and one for the ground tile to match it, but it works just fine.
However, it'll only look right if you replace a dirt (bumpy) cliff with another dirt cliff, or a man-made (smooth) cliff with another man-made cliff.
When I replaced the dirt cliff tile for the Dungeon tileset, I used TerrainArt\Dungeon2\GDirt.blp for the texture, and ReplaceableTextures\Cliff\Cliff0.blp for the cliff itself. I think that second path is the same for the second texture of all tilesets; it'll replace whatever tileset's cliff that you're using. I'm not 100% sure on that, though.
Good luck.