When Omeka detects that a user has installed a new version of the plugin, it automatically deactivates the plugin and presents an "Upgrade" button. This hook runs when a superuser clicks the "Upgrade" button. It is primarily used to migrate database tables and other data stored by older versions of the plugin.
stringold_versionThe previous version that was installed and is being upgraded from.
stringnew_versionThe new, current version of the plugin.