I have a REALLY old Owncloud instance (5.0.13) running on an even older server. I have a new server from Linode I would like to install Nextcloud on. Obviously there’s no published upgrade path from that far back.
If I was only interested in copying the contacts and calendars, could I simply save the calendars as .ics files and import them on the new install, and save my contacts from the old server as a vcf file and import that on the new server, would that work?
Anything I should be wary of, or data I should expect to lose doing that?
You would have to run all major upgrades on ownCloud first (OC 5 -> OC 6 -> OC 7 -> OC 8.0 -> OC 8.1 -> OC 8.2 -> OC 9.0 => NC 9.0 -> NC 10 -> NC 11 -> NC 12). You can get the old ownCloud versions here: https://owncloud.org/changelog/server/, for Nextcloud https://nextcloud.com/changelog
Then the upgrading party is quite some work. Your idea of exporting and then importing the data seems the best approach. For only contacts and calendars this was already recommended in the past. In this case, probably nobody tested the procedure for this combination of versions. In the past there were sometimes formatting problems with single contacts. So I recommend to keep the old setup (at least on a backup machine) until you are sure that the import has worked probably.
Additonal backup idea: use a carddav and caldav client and use them to export data as well. Then connect to your new Nextcloud setup and import your data again. A program of the same version should be able to export and import its own data.
If you backup contacts and calender entries you will lose everything else Expect devices to synchronise again. All settings in the interface will be lost, all other app data.