How to downgrade python version on CentOS?

Instead of literally downgrading, I would consider virtualenv. It lets you create isolated python environments, including the specific python version and the modules.

You can always install a different version of Python using the -altinstall argument, and then run it either in a virtual environment, or just run the commands with python(version) command.

A considerable amount of CentOS is written in Python so changing the core version will most likely break some functions.