There are rare occasions when you want to re-index all your site's content in Solr. Such occasions include:
- Major Drupal version upgrade (e.g. from Drupal 6.x to Drupal 7.x).
- Changing your Solr schema to include more search criteria.
- Upgrading your Solr server to a new major version.
- Moving your Solr server from an old server to a new one.