I wanted to replicate an old git repo, in which I used as author my old university-work email, but using my current gmail account as email. Changing it for both commiter and author for all commits involves rewriting the whole repo. It’s not difficult at all, but better work on a copy of the repo, and remember that if you shoot yourself on the foot you can always get back by doing git reset --hard HEAD.

The rewrite can be easily done by using the filter-branch tool applied to all branches at once:

git filter-branch --env-filter '
 GIT_COMMITER_EMAIL=<new.email@gmail.com>' --all

This is the nuclear way, it will rewrite absolutely all commits. This won’t work straight if you have code from collaborators, then you’d lose their emails after the unconditional rewrite.