No matter what units you are exchanging to and from, the number of unit cubes will remain constant. For example: If I had a cube who's side was 10 km long, the number of unit cubes would be the volume of the cube which in this case is 1000 km³. So there would be 1000 unit cubes. Then, if I changed the side from 10 km to 10 in, and find the volume, it would be 1000 in³, meaning there are 1000 unit cubes! So the answer to your question is: The number of unit cubes remains constant