In the end it is just an electric motor (I'm assuming it is a DC one), just open the dremel up and swap the wires over on the motor. There is a possibility that there are 3 wires (one for speed control), if so DON'T swap anything, I'd have to have a look at it. If your handy, you could wire a switch so that you could do this on the fly.
You would also need to check that the cutting tools are not directional before use.