https://lazyadmin.nl/home-network/migrate-unifi-controller/

Note

On (new) Key Gen 2 the restore are NOT done on the first system settings page (/settings/system).

Restore are on another page: /settings/updates > Click Network > Open

This button will open the settings page for the network on the new Key Gen 2, and from here (/network/default/settings/system) the Restore > Restore Backup > Import button will actually work, and the Gen 1 Key .unf file can be imported and used for restore.

Steps to migrate from Unifi Cloud Key Gen 1 to Gen 2:

1. Startup new key on your network, and do basic initalization (I used mobile app, make sure your mobile phone are not connected to a Wifi, and enable Bluetooth)
2. Keep both Keys on network
3. Login to new Key and old Key using a computer
4. Upgrade the all possible options for software and firmware on both new Key and old Key to latest version
5. Using computer: On old Key, go to Settings > Sites. Scroll all the way to the bottom, and click the grey button Export sites. Follow the instructions, i will type them down here:
6. Your old config .unf file will be downloaded to your computer
7. Restore that file into your new Key (/network/default/settings/system)
8. Migrate all devices
9. Wait 5 minutes
10. Forget all devices on your old Key
11. Shut down your old key from Settings
12. Done. For convinience: Adjust the IP of your new Key to match the IP of your old Key

original post https://community.ui.com/questions/Migrate-from-Unifi-Key-Gen-1-to-Gen-2/ee76ecb0-3434-4ec7-a41e-400b408691de

2nd possible solution

1) Download a backup of your current controller.
2) Shutdown the old controller.
3) Plug the new UCK in.
4) Configure the UCK UI
5) Set a static IP ( same as old controller )
6) Import the backup.

see also

https://help.cloudunifi.com/unifi/migrating-sites-with-site-export-wizard/

https://help.ui.com/hc/en-us/articles/360008976393-UniFi-Backups-and-Migration#:~:text=Restore%20the%20backup%20file%20to,online%20in%20the%20new%20application.