What would happen to the users of my app below 9.3?
They wouldn't receive any more updates to the application.
What would happen to someone who would like to download my app but is below 9.3?
There's an option in iTunes Connect you can enable in order to allow them to download the most recent version of your application that supports the version of iOS they are using. If you don't enable this, they are shown an error message saying that your application isn't compatible with their version of iOS.