According to the Owncloud 9 release notes, you can't skip the 9.0 upgrade if you want to migrate calendar and addressbook entries to the new CalDav and CardDav code that ships as a core function of owncloud. Apparently the migration of this data is in 9.0.x only, and not 9.1 or later versions. Does the move from 8.2.4 to 10.0.9 include the 9.0.x migration step as part of the upgrade?

I just tried to revert to 8.2.4. Owncloud 8.2.4 won't run, presumably because of the php changes that went with owncloud 10 (there are php errors in syslog). Doing a yum remove owncloud-files only removes the app-owncloud, app-owncloud-core, and owncloud-files and leaves the php change intact. I guess I'll have to remove the php changes manually; hopefully that will revert everything back to php 5.4 (or whatever it was) and owncloud 8.2.4 will work again.