XigmaNAS

I have installed the OS to an old HDD I had in spare.
Now I was figuring that a USB-Stick/Flashdrive would be even better… Can I switch the configuration of the OS to a USB-Stick or Flashdrive? Is there anything I have to be aware of? The system is live and in use. Any data lost is not acceptable

Save the configuration, then install the (embedded) system on that USB stick and restore the config.
That's all - usually.

if you are moving to other hardware
setting the LAN nic and IP address to first load in the previously saved config and again to change the restored config to the new hardware