Explanation
https://aws.amazon.com/blogs/networking-and-content-delivery/adding-http-security-headers-using-lambdaedge

Simply upload the new version of your application and let your deployment service (AWS Elastic Beanstalk, AWS CloudFormation, or AWS OpsWorks) deploy a new version (green). To cut over to the new version, you simply replace the ELB URLs in your DNS records. Elastic Beanstalk has a Swap Environment URLs feature to facilitate a simpler cutover process.
https://d0.awsstatic.com/whitepapers/overview-of-deployment-options-on-aws.pdf

Explanation
As your infrastructure grows, common patterns can emerge in which you declare the same components in each of your templates. You can separate out these common components and create dedicated templates for them.
That way, you can mix and match different templates but use nested stacks to create a single, unified stack. Nested stacks are stacks that create other stacks. To create nested stacks, use the AWS:: Cloud Form ation::Stackresource in your template to reference other templates.
For more information on nested stacks, please visit the below URL:
* http://docs^ws.amazon.com/AWSCIoudFormation/latest/UserGuide/best-practices.html#nested Note:
The query is, how you can automate a stack over the period of time, when changes are required, with out recreating the stack.
The function of Nested Stacks are to reuse Common Template Patterns.
For example, assume that you have a load balancer configuration that you use for most of your stacks. Instead of copying and pasting the same configurations into your templates, you can create a dedicated template for the load balancer. Then, you just use the resource to reference that template from within other templates.
Yet another example is if you have a launch configuration with certain specific configuration and you need to change the instance size only in the production environment and to leave it as it is in the development environment.
AWS also recommends that updates to nested stacks are run from the parent stack.
When you apply template changes to update a top-level stack, AWS CloudFormation updates the top-level stack and initiates an update to its nested stacks. AWS Cloud Formation updates the resources of modified nested stacks, but does not update the resources of unmodified nested stacks.

Explanation
This deployment technique is given in the AWS Whitepaper

As you scale up the green Auto Scaling group, you can take blue Auto Scaling group instances out of service by either terminating them or putting them in Standby state. Standby is a good option because if you need to roll back to the blue environment, you only have to put your blue server instances back in service and they're ready to go.14 As soon as the green group is scaled up without issues, you can decommission the blue group by adjusting the group size to zero. If you need to roll back, detach the load balancer from the green group or reduce the group size of the green group to zero. For more information on Blue Green deployments, please visit the below URL:
* https://dOawsstatic.com/whitepapers/AWS_Blue_Green_Deployments.pdf