Whether you are migrating from a Cloud Key, local UniFi install, or other cloud UniFi server, we recommend following this guide to migrate each of your UniFi sites to HostiFi.
UniFi - Migrating Sites with Site Export Wizard
Why not do a full backup/restore of UniFi?
We typically recommend using the site migration wizard instead of UniFi's backup/restore, here's why:
A backup/restore migrates all sites and device configurations, but the devices themselves are not migrated, so you would need to have them all connected to a DNS name, and change the DNS name to the new server IP. The migration wizard is easier because it allows you to migrate the devices to the new server without changing any DNS records.
A backup/restore only works one time. If you have multiple UniFi servers, the only way to migrate them into the one HostiFi server is to use the site migration wizard.