I had a directory full of CEL files, which I wanted to bring to lower case for consistency with other CEL files I had within that project.

To rename all files in a specific directory on a Debian based system you can use:

rename 'y/A-Z/a-z/' *

Or if you only want to change the extension:

rename 's/\.CEL.GZ/\.cel.gz/' *

Note:

The rename command on Red Hat based systems is the util-linux rename.