Are you on linux? Somthing like this should help:
find /var/www/html/dokuwiki/data -type f -exec sed -i 's/Old Username/New Username/g' {} +
But be careful and try on a copy of your wiki first. I think you will have to execute the command as root, so you damage your wiki if your search pattern doesn't work correctly. I becomes a bit more complicated if special chars are in your text.
