It's a bit messy, but I have a partial solution, but some things aren't working still.
If you do a mysqldump of the existing owncloud database, you can then run sed against the output file to export the following tables;
sed -n -e '/DROP TABLE.*'oc_groups'/,/UNLOCK TABLES/p' owncloud-sqlbkp_20170630.bak > owncloud-oc_groups.sql
If you've already created an empty (new) OC database on a new server, you can them import these tables into the new installation. A quick refresh, and you'll see all the user accounts and groups intact. Things I haven't worked out yet are;
Passwords: The passwords are in the oc_users sql table, but they don't seem to import properly. Could just be the difference between the older OC versions and 9.1.5. These can be reset through the UI, or using occ.
Quotas: No quotas are carried over, so need to be re-set
Once the user accounts are across, you can then scp the files from old to new server, then run the occ command against them to re-scan/re-fresh the OC file ownership information (make sure your chown all the data tree to the owner of your web server first - in my case 'apache')
sudo -u apache php /var/www/html/owncloud/occ files:scan --all
This sort of works, but it could do with the fine tuning. It would be great if passwords could be carried over, as on our main OC server we have over 100 accounts.
Work in progress, but it would be good if there was an official way that didn't involve 15 intermediate upgrades!