carbon is not necessary in a filter. It's just kind of a cosmetic thing...carbon clears up colors,odors, and sometimes cloudiness. Most people who seriously keep tanks don't use it unless they need to remove meds, and I do the same. There are some people who say that old carbon in a filter can actually leach toxins over time. IDK about that.
Since a whole lot of your good bacteria are in the filter, you don't want to replace that insert. If you'd like to remove the carbon, just cut a hole in it and swoosh the carbon out in old tank water, as much as you can.
I don't replace my (carbon free) filter inserts until they are falling apart. A "dirty" look, without being really slimy,is actually good. That means your bacteria are there, doing thier jobs.