A Saver Day Pass is a pass valid on all public transit in Switzerland. Not only can you take a different train, you can also change your plans completely and go to somewhere else. And in Bellinzona you can take local public transit. If after your visit to Bellinzona you feel like going to Lugano you can do that…
It is an all you can eat pass for the whole country.
Your reservation you will have lost if you change your plans though. But reservations are not needed anyway.