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Changing author (or any other commit attribute) in existing commits

From this SOF Q&A. Linked page was showing a scenario to change committer and author's name and email from old value to new value:

git filter-branch --commit-filter '
    if [ "$GIT_COMMITTER_NAME" = "<OLD_NAME>" ];
    then
        GIT_COMMITTER_NAME="<NEW_NAME>";

        # ...
        # modify other attributes if any, then...

        git commit-tree "$@";
    else
        git commit-tree "$@";
    fi' HEAD

Comment in the answer was mentioning git env-filter will change all commits, but the one shown above allows conditional behaviour.

Editing hunk while adding commits

There are couple articles mentioning it, including the git add man page. Section EDITING PATCHES in the manual has detailed explanation.