If you are migrating from an earlier version than 4.0, and your database contains statistics collected using a version earlier than 2.2, you will probably encounter an issue during our automated database migration. To avoid that issue, you will need to clean up your statistics by executing some SQL commands described in issue XWIKI-8129. Be sure to apply these before attempting the migration. If your are in doubt, apply it, these could not hurt. If you don't, you will face duplicate IDs errors during the migration process, and it will fail, preventing you to run your wiki.

Always make sure you compare your xwiki.cfg and xwiki.properties files with the newest version since some configuration parameters were added. Note that you should add xwiki.store.migration=1 so that XWiki will attempt to automatically migrate your current database to the new schema. Make sure you backup your Database before doing anything.