Issue

• RHEL7 version of usermod states (in its manpage):

  -g, --gid GROUP
  The group name or number of the user's new initial login group.
  The group must exist.
  
  Any file from the user's home directory owned by the previous
  primary group of the user will be owned by this new group.
  
  The group ownership of files outside of the user's home
  directory must be fixed manually.
  

• Some use cases (ie. system users with '/' as home) require just changing the GID value in /etc/passwd, without actually touching any files in a home directory