In vanilla Minecraft, using the hoe on a block of coarse dirt, turns it into normal dirt. However, a farming turtle placed on top of a block of coarse dirt will report "Unbreakable block detected" when calling digDown(). I believe this is a bug, as I think the turtle using the hoe on coarse dirt should turn it into dirt as well.

My apologies, It didn't work due to me not understanding how the farming turtle works. There should be an air block between the turtle and the coarse dirt, in which case it turns it into normal dirt as expected.