public KnownVersion getDataVersionIfOld() {
    final KnownVersion version = dataVersion;
    if (version == KnownVersion.CURRENT) {
      return null;
    }

    // version changed so return that for VersionedDataStream
    return version;
  }
If this OpLog has data that was written by an older version of the product, then return that
{@link KnownVersion} else return null.
geode-core/src/main/java/org/apache/geode/internal/cache/Oplog.java