AWS CloudFormation launches a new parameter for the CreateChangeSet API called ImportExistingResources that streamlines the process of importing resources into stacks. When you deploy ChangeSets with the ImportExistingResources parameter, CloudFormation automatically imports the resources in your template that already exist in your AWS account. CloudFormation uses the custom names of resources in your template to determine their existence. With this launch, you can reduce the manual effort of import operations and avoid deployment failures because of naming conflicts.
Source:: Amazon AWS