public FieldValue optionallyUpgrade(Field field, FieldValue fv) {
        return optionallyUpgrade(field.getDataType(), fv);
    }
If the FieldValue is a StructuredFieldValue it will upgrade to the concrete type
@param field to upgrade
@param fv value to upgrade
@return fv or upgraded fv
docproc/src/main/java/com/yahoo/docproc/AbstractConcreteDocumentFactory.java