Place the following in a command file, run it within your repo:
#!/bin/sh
git filter-branch --env-filter '
an="$GIT_AUTHOR_NAME"
am="$GIT_AUTHOR_EMAIL"
cn="$GIT_COMMITTER_NAME"
cm="$GIT_COMMITTER_EMAIL"
if [ "$GIT_AUTHOR_EMAIL" = "timp@paneris.org" ]
then
an="Tim Pizey"
am="timp21337@paneris.org"
fi
if [ "$GIT_COMMITTER_EMAIL" = "timp@paneris.org" ]
then
cn="Tim Pizey"
cm="timp21337@paneris.org"
fi
export GIT_AUTHOR_NAME="$an"
export GIT_AUTHOR_EMAIL="$am"
export GIT_COMMITTER_NAME="$cn"
export GIT_COMMITTER_EMAIL="$cm"
'
Then
git push -f origin master
Note that this process is very slow so do not repeat for every change: add all the changes you want to make and perform in one pass.