The ownCloud 1-click image is just an ownCloud instance pre-installed on a Droplet. It isn't any different from a regular ownCloud instance that is manually installed on any other Linux server.
Migrating an ownCloud instance to NextCloud is very simple. Start off by taking a backup of your files and database:
sudo cp -r /var/www/owncloud ~/owncloud_backup_pre_migration
sudo mysqldump --lock-tables -u root owncloud > owncloud_backup_pre_migration.sql
Delete everything but the
data directories. We'll do that by creating a new directory and moving the directories that we want to keep to it.
sudo mkdir /var/www/nextcloud
sudo mv /var/www/owncloud/config /var/www/nextcloud/
sudo mv /var/www/owncloud/data /var/www/nextcloud/
Then, download and extract Nextcloud into the new directory. You can grab the latest release's download URL from this page.
wget -O nc.tar.bz2 https://download.nextcloud.com/server/releases/nextcloud-9.0.53.tar.bz2
tar vxjf nc.tar.bz2
sudo apt-get remove owncloud
Now, delete the old owncloud directory and rename the new one to owncloud:
sudo rm -r /var/www/owncloud
sudo mv /var/www/nextcloud /var/www/owncloud
Finally, trigger the upgrade:
sudo -u www-data php occ upgrade
You should now be running a working nextCloud instance. Good luck, and let me know how it goes!