Whats the best way replace instance without downtime?
The best way IMHO to replace your instances with zero downtime is to use URL swap; which is a simple operation of exchanging CNAMEs between two ready environments.
Read more at AWS
Deploying Versions with Zero Downtime (Short version)
Copy Alpha environment configuration.
Create Omega environment based on Alpha configuration.
Ensure that Omega environment is functioning as expected using your hosts file (obtain IP by pinging your Elastic Beanstalk provided URL i.e omega.elasticbeanstalk.com).
Test Omega and ensure that Alpha monitoring reports zero traffic.
Terminate your Alpha instances in the AWS Certified EC2 interface forcing a rebuild of the instances.
Wait for the rebuild to complete and swap back the URL's
Terminate the Omega environment
Help me on this!