If you'd like to migrate your #AddressBooks and #calendars from somebody else's platform to a self-hosted or self-managed one, but there's no "migration assistant":
Check whether both platforms support the #CalDAV and #CardDAV standards. If they do, look into #vdirsyncer https://vdirsyncer.pimutils.org
We'd recommend to test-migrate your existing data to a local filesystem storage first, and swap in your new storage only when syncing to the test storage worked without errors.
To protect yourself against migration blunders, mark your source storage (platform) as read_only
and let it "win" in your conflict_resolution
setting.
For address book pairs, you can also sync the "displayname" and "description" metadata
– for calendar pairs, additionally "color" and "order".