i did loose access to the device and wan.
this is the reason i went on hard reboot cycle after, and eventually after 3 or 4 tries, it reverted back (but not hard reset) but i am not sure what made it happen.
come on, changing subnet is not something extraordinary and i would call that a major bug if router ends up bricked.
Now thinking of it, the reverting may have partialy failed because of conflit with wan network that has same subnet than the lan one reverted too. ( and obviously the very reason why i wanted to change it on the first place ... )
In any case the "feature" would gain to be enhanced for subnet changes...
Then that means you had same subnet on LAN and WAN and no routing took place, that's why you lost Internet.
Perhaps, a lot of people are having snags changing LAN subnet in LuCI now, I do recognize that. In the past, people would lock themselves out (at that point) if there was an error, though.
i removed the wan entry to avoid any interference, and proceeded again changing the lan ip subnet.
after the 30 seconds, the conf is trying to revert (same image as above with loading wheel) but does not complete.
At that point the router does not provide any DHCP anymore and does not respond. Only a reboot will bring it back.
So there is something not working as intended with feature /luCI in my setup for basic lan subnet change.
Now i retried it again with advise above "to renew DHCP lease and reconnect within the 30 seconds time slot". And this worked OK.