A version number should be specified on the root element of the DFDL schema, and this version number should end up in the output XML representation as an attribute of the root element.
Similarly, when reading XML to unparse, the version attribute should be read (if present), and compared against the version number in the schema.
TBD: how to tolerate compatible versions that are not exact matches.
All this would also apply to other output formats that we do in the future. (e.g., JSON)