Do not attempt to keep settings across the upgrade and don’t try to restore a backup. Doing so will cause major problems. Your router will be in a completely default state after the upgrade - no config files will remain from your previous setup.
That said, a backup will be useful and you can usually restore certain specific files directly - firewall and dhcp are fine. Don’t try to restore network or wireless files, though. Just use them as a reference as you recreate your config.