title: Can protobuf data types be upgraded?
body: <p>I'm new to Protobuf and so don't know if this is possible. If in a proto file I have a <code>fixed32</code> datatype, can I upgrade it to <code>fixed64</code> such that all <em>existing</em> <code>fixed32</code> values can also be read (i.e. backward compatibility) ?</p>
  
  <p>If not, what's the best way to perform such an upgrade ?</p>