code: void initializeDistributedUpgrade() throws IOException {
      if(! upgradeManager.initializeUpgrade())
        return;
      // write new upgrade state into disk
      writeAll();
      LOG.info("\n   Distributed upgrade for NameNode version "
               + upgradeManager.getUpgradeVersion() + " to current LV "
               + layoutVersion + " is initialized.");
    }
docs: Initialize a distributed upgrade.