How to change the teaser for the front page articles
You need to Edit and Save again the articles to take effect.

I know can be painful if you have hundreds of nodes. If you google about it you will find scripts to do it like this
http://befused.com/drupal/script-update-drupal-teasers

Or use the module retease http://drupal.org/project/retease

I advice you to take a database dump before testing these solutions, just in case.