One of the comments in the thread links to a useful writeup for the AWS side:

https://www.knowmatic.app/how-to-migrate-aws-ec2-instance-fr...

I didn't find such great docs on the Cloudflare part, but some of these were useful:

https://www.reddit.com/r/selfhosted/comments/14d16lt/if_i_on...

https://community.cloudflare.com/t/the-page-isn-t-redirectin...

https://stackoverflow.com/questions/4616521/nginx-configurat...

https://developers.cloudflare.com/support/troubleshooting/cl...