On my tests, none was lost so far. But I guess it all depends which features are you using that is dependent on the router os version.
On my setup, here’s what I have used so far and no config was lost.
Hotspot
Policy routing for 2 gateways
PPP client for the USB 3G dongle
OpenVPN client
2 SSID
Bridged LAN
Activated radius external server
firewall, srcnat, dstnat
custom scripts and scheduler