You also need to adjust the alarm threshold to accommodate for the new shard capacity automatically. For this example, update the alarm threshold to 80% of your new capacity (or 3200 records per second) by setting a CloudWatch alarm with an action to publish to a SNS topic when the alarm is triggered.
You can then create a Lambda function that subscribes to this SNS topic and executes a call to the new UpdateShardCount API operation while adjusting the CloudWatch alarm threshold. To learn how to configure a Cloudwatch alarm, see Creating Amazon Cloudwatch Alarms. For information about how to invoke a Lambda function from SNS, see Invoking Lambda Functions Using Amazon SNS Notifications.
This is pretty cool.