Worldedit Everything under grass replacen?

I love building in Minecraft and because I love terraforming, I specialize in it. Only I have a problem: every time I take a brush that is placed on grass, everything under the top layer of the grass does not become earth. So if you go into a normal MC world, it's under the grass, but that's not the case with me. How can I replace it so that only that under the top layer of grass becomes dirt?

You can use // overlay grass to put a layer of grass over the selected area if you mean that.