At the easiest, changing from the CPU version of ND4J / ND4S should be as simple as changing the dependency in your build.sbt:
libraryDependencies += "org.nd4j" % "nd4j-cuda-8.0-platform" % "0.9.1"
libraryDependencies += "org.nd4j" % "nd4j-native-platform" % "0.8.0"
If you get a module resolution error, you need to delete ~/.ivy.
failed cuda;8.0-6.0-1.3!cuda.jar
I had to switch back to 2.11:
scalaVersion := "2.11.11"
If you let Intellij fill in references for you, you may need to remove bad references to NDArray in the cpu namespace (use INDArray instead).
import org.nd4j.linalg.cpu.nativecpu.NDArray