Git: Change author name in all previous commits

On any branch, or trunk:

git filter-branch --commit-filter 'if [ "$GIT_AUTHOR_NAME" = "Josh Lee" ];
  then export GIT_AUTHOR_NAME="Hobo Bob"; export GIT_AUTHOR_EMAIL=hobo@example.com;
       export GIT_COMMITTER_NAME="Hobo Bob"; export GIT_COMMITTER_EMAIL=hobo@example.com;
fi; git commit-tree "$@"'

(source)