Yep, I did it on two bikes with the Nexus 3 speed just a few months ago...not too difficult.

There's a little screw on the shifter face that allows you to remove the face and expose the cable attachment point. I stared at it for a while, and decided to set the shifter for max cable let-out and just gently pry the cable head out of the track it lies in. Replacement is simply you fitting the head of the shift cable into that same spot and running it through all the housing and attaching it to the Nexus piece that connects to the hub (forget what it's called).

I didn't have an issues - just use good sense and don't try to force anything and you should be fine.