I recently ran into a situation where I had enabled HDFS HA and later had to change the value of dfs.nameservices. So basically during HA setup I set the value for dfs.nameservices to "MyHorton", but a couple hours later realized I should have used "MyCluster" instead.
This article explains how you can change the dfs.nameservices value after HDFS HA has been enabled already.
Background: What is the purpose of dfs.nameservices?
Its the logical name of your HDFS nameservice. Its important to remember that there are several configuration parameters that have a key, which includes the actual value of dfs.nameservices, e.g. dfs.namenode.rpc-address.[nameservice id].nn1