This precise question is also answered in the git manual:
$ git filter-branch --env-filter 'if test "$GIT_AUTHOR_NAME" = "foo"; then GIT_AUTHOR_NAME="bar"; fi'
This filter may be used if you only need to modify the environment in which the commit will be performed.
This will rewrite all the commits though. For rewriting only a selected range, add it at the end
$ git filter-branch --env-filter '.....' abcdef..HEAD
If you run it a couple of times you'll get an error, "Cannot create a new backup. A previous backup already exists in refs/original/". Just add
--force to the command.
Author and committer are separated in git. The author is the person who wrote the patch, whereas the committer is the person who applied the patch. So if you send a patch to someone else you are the author, and the project member that merged it is the committer.