Nextcloud
https://nextcloud.comTry Nextcloud now. Share and collaborate on documents, send and receive email, manage your calendar and have video chats without data leaks. As fully on-premises solution, Nextcloud Hub provides the benefits of online collaboration without the compliance and security risks. Nextcloud 22 introduction and overview from Nextcloud GmbH on Vimeo.
Migration: ownCloud vs Nextcloud – Nextcloud
https://nextcloud.com/migrationThis tool supports ownCloud 8.2 to ownCloud 10.5, migrating to the appropriate Nextcloud release automatically. There can be PHP version mismatches. As of this writing we have developed a direct migration from ownCloud 10.5 to Nextcloud 20.0.4, avoiding most conflicts but for newer releases things will become complicated again.