Specifies whether an Auto Scaling group and the instances it contains are replaced during an update.
During replacement, AWS CloudFormation retains the old group until it finishes creating the new one. If the update fails, AWS CloudFormation can roll back to the old Auto Scaling group and delete the new Auto Scaling group.
While AWS CloudFormation creates the new group, it doesn't detach or attach any instances. After successfully creating the new Auto Scaling group, AWS CloudFormation deletes the old Auto Scaling group during the cleanup process.
When you set the WillReplace parameter, remember to specify a matching CreationPolicy. If the minimum number of instances (specified by the MinSuccessfulInstancesPercent property) don't signal success within the Timeout period (specified in the CreationPolicy policy), the replacement update fails and AWS CloudFormation rolls back to the old Auto Scaling group.