This is just "thinking out loud" as I don't have the exact
syntax of the code available, only the "theory behind the thought"

First -> find out which folder of your system the executable
is stored (maybe it's /usr/bin or /usr/lib)...

Then, use synaptic or command line to completely remove gnochm.
--- BEFORE YOU DO, pleas see below @ "Why this might not work"

Once gnochm is removed, you can then create a symlink (or soft link) that
if "gnochm" is requested , point instead to kchm in the same folder where the
gnochm executable is or was.

Why this might not work. Once gnochm is removed, it may (possibly)
disappear from your Preferred Applications list as a chm file handler.

Possible workaround... once you do find the executable for gnochm,
then (as sudo-root) rename the file. This should prevent gnochm from launching
under its given/natural name.

Since the system still knows of gnochm, you can then create the symlink.

Again, I don't know if it will work or be the most practical solution.