Moving and migrating from multiple Owncloud installs to single Nextcloud

The small company where I’m working currently has two OwnCloud installs at two different locations.Both are running ownCloud 9. I have been tasked with migrating these into a single Nextcloud setup.

ownCloud 1 running 9.0.2 and is configured on a Synology NAS. ownCloud 2 is running 9.1.1 and is configured on a Ubuntu server, and is in a different location on a different network. Ideally we would be able to move the users data from OwnCloud 1 to the ubuntu config and then update that server keeping both the original users data, and the one that has been moved.

The upgrade from ownCloud 9 to Nextcloud I think I can handle, but moving the data and users from the synology is more difficult, if at all possible. Any tips on how I can best go about this are highly appreciated!