A few days ago Nextcloud was officially announced and the Nextcloud team crafted a new release rather quickly.

The following steps were required to migrate from ownCloud 8.2.5 to Nextcloud 9.0.50:

• Create a backup of the current ownCloud installation.
• Install Nextcloud by following this migration guide.
• Perform the actual upgrade for all apps: $ php occ upgrade --no-app-disable.
• For each third party app: disable it in the WebUI, reload, enable it in the WebUI.
• Recover missing files: $ php occ files:scan --all.
• Check if the calendar/contacts migration worked as expected.
• Update the Nginx configuration.
• Remove the old ownCloud installation.
• For each mobile phone: replace the ownCloud app with the Nextcloud app.

Have fun.