Seconded. The git blame-someone-else tool is just using git rebase and git commit --amend internally to alter one specified commit.
git filter-branch is perfect for this kind of wholesale revision. filter-branch is essential for tasks like: open-sourcing repos that need some kind of cleanup, massaging repos generated by a VCS migration tool, etc. For example, years ago I participated in the move of a large CVS repo to git; there was significant filter-branch post-processing required to create an acceptable baseline)