Thank you Seungwon Choi. I'm glad this article helps you.
To achieve the 3 tier architecture set up I pictured in the first diagram needs a replatform. Rehost is simply changing the underlying infrastructure where the workload is running on. One example there is where I show RDS, which is a managed RDBMS: if it is rehost RDS won't be used and instead, we would migrate the on-prem RDBMS to run self-managed on EC2 instances. The other way I like to think about rehost vs replatform is if you can do the entire move with just VM Import/Export and some simple config changes, then the migration can be considered as rehost, otherwise it's a replatform. But again this is just a general guidance, not a hard rehost vs replatform line.
I hope this helps. If you have any topic you'd like me to cover, please follow me and drop me a line :)