The description of "usermod -g" in the manpage does not accord to its real behaviour. Actually, this has not been implemented, so you have the rather difficult task to write the implementation for this.
You can use code from the other tools which deal with user and group modification, maybe you will find a place where nearly the same work has already been done.

http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=41049