Decreasing Read Capacity
To decrease read capacity, delete one or more read replicas from your Redis cluster with replicas (called replication group in the API/CLI). If the cluster is Multi-AZ with automatic failover enabled, you cannot delete the last read replica without first disabling Multi-AZ with automatic failover. For more information, see Modifying a Cluster with Replicas.
Due to the current limitations on Redis (cluster mode enabled), to scale the number of replicas in the node groups of a Redis (cluster mode enabled) cluster with replicas, you must create a new Redis (cluster mode enabled) cluster with the new number of replicas and then delete the old replication group.
For more information, see Deleting a Read Replica.