If you change either terminal or radio address on a terminal using cellular base, even before he saveing (F4) or reseting (o-b-r) it, the controller immediately reports a tnum error. This can hang a terminal session.
If you are changing multiple terminal numbers and radio addresses, in extreme cases you can actually lock up the whole controller and have to reboot it. Therefore, when you are dealing with production systems where you may not be able to reboot the controller, you need to break communication between the terminal and controller (i.e. change to an unused frequency, get out of range, set radio->tests->online=N) and make all changes. Then you can re-establish communications.