With Amazon EMR release 5.27.0, you can now reconfigure your multi-master clusters on the fly, without needing to recreate your cluster. EMR 5.23 added the capability to run multiple master nodes, helping you protect your long running clusters that run important data pipelines, streaming applications, or HBase clusters from the potential loss of a master node. EMR also gives you the ability to reconfigure running clusters, giving you the ability to change configuration using the EMR console, CLI, and SDK. With EMR release 5.27.0, you can now reconfigure your running multiple master node clusters, allowing you to easily change the EMR application configuration in your running cluster without having to recreate it.
from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/10/amazon-emr-adds-support-for-spark-2-4-4-flink-1-8-1-and-ability-to-reconfigure-multiple-master-nodes/