You are right. You’d have this behaviour you desire once bluegreen deployments for AppsV2 is implemented: Any update on when bluegreen deploy strategy will be ready? - #2 by kwaw
If you’re up for it, you could accomplish a roughly similar behaviour by flyctl m destroy <old-machine> post successful flyctl m run <new-image> (or using equivalent Fly Machine APIs).
If you’re running the main process as root, it can capture the kill_signal (ex) and gracefully shutdown (if needed) within specified kill_timeout secs (ex).