HDFS-6031 re-added the "-rollingUpgrade started" option for the NameNode, but the help text still doesn't include it:
Usage: java NameNode [-backup] |
[-format [-clusterid cid ] [-force] [-nonInteractive] ] |
[-upgrade [-clusterid cid] [-renameReserved<k-v pairs>] ] |
[-rollingUpgrade <downgrade|rollback> ] |
NN running newer software rejects loading the fsimage during rolling upgrade.