All modern aircraft are assigned a unique ICAO 24-bit address or (informally) Mode-S "hex code" upon national registration and this address becomes a part of the aircraft's Certificate of Registration. Normally, the address is never changed, however, the transponders are reprogrammable and, occasionally, are moved from one aircraft to another (presumably for operational or cost purposes), either by maintenance or by changing the appropriate entry in the aircraft's FMS system.
That's one of the worst explanations I've ever seen.
Firstly, it's by no means uncommon, as we all know, for an aircraft's Mode S address to change. Although the ICAO guidelines say that this should only happen when an aircraft is re-registered in another country (because each country has a separate allocation of addresses) many countries such as the US, Canada, France, Germany, etc, will allocate a new Mode S code if, for example, an aircraft changes from one N-number to another N-number.
And as far as I recall from my aircraft maintenance days, the only way of moving a transponder or any other component from one aircraft to another is by taking it out, walking across the hangar and re-fitting it to the recipient aircraft. You can't just push buttons on the FMS and expect the component to change itself :-)
