A GNU/Linux distro (sorry, no Windows support)
The non-free package unrar-nonfree (check unrar-nonfree)
The free package zip. Install it with https://packages.debian.org/source/sid/unrar-nonfree.
Python (checked with Python 2.7 and Python 3.3)

Usage

To convert a single CBR file use this (the script can't handle filenames with spaces):

./cbr2cbz.py <cbr_filename_without_spaces>

To convert an entire directory with CBR files in int use:

cbrfolder2cbz.py <directory>

The script will put CBZ files in the same directory

Introduction

CBR files use RAR format, a non-free format. If you use a GPL comics reader in your tablet it's likely it will have problems when reading CBR files. Convert them to CBZ with this script.