How do you replace grip shifter cables?

I was able to unscrew the grip shifter from the handlebars. Then I tried prying the grip part away from the shifting mechanism with a flathead screwdriver. This led to me breaking a plastic portion of the mechanism. I ended up buying a whole new grip shifter which has the cable pre-installed, but I would have preferred to just replace the cable. Is there a reasonable way to swap out an old cable on a grip shifter? Thanks.