To change the name and/or email address recorded in existing commits, you must rewrite the entire history of your Git repository.
Quoted from github :
This action is destructive to your repository’s history. If you’re collaborating on a repository with others, it’s considered bad practice to rewrite published history. You should only do this in an emergency.
I had to run this script more than once, because I am pretty bad at managing my identities. I added the '-f' option