Yes you can migrate your license to another machine. Just unplug your old server or simply shutdown so the Deck cannot see that server, and then the option to Disconnect your old server will appears. All you have to do is click that button, and your license is free to use again at another machine.
On top of that, your old server will still be functional, all your config and files still intacts. You can access it via TrueNAS UI. It just no longer being managed by HexOS.
arvinpw's post in Lift and shift from one server to another? was marked as the answer