Changing an SVN commit message retroactively involves two steps. First, the repository must have a hook enabled. Then the svn client must issue a propset command.
On the subversion server:
cd /var/www/svn/myrepository/hooks
cp pre-revprop-change.tmpl pre-revprop-change
chmod u+x pre-revprop-change
chown apache:apache pre-revprop-change
Line-by-line explanation: