My app has a registration screen that is only presented once, and after registration is completed it is never shown again.
If my startup screen is a screenshot of the app then I would need to provide two screenshots and for the correct one to be initially displayed when the app launches.
Is this possible?
No, it is not currently possible. You cannot access the Default.png (or whatever you’ve named it instead) from your app to update it.
If it were me, I would set the startup screen to the one that is appropriate long term. You could also theoretically update your registration screen to be a bit closer to the look and feel of the main app screen, so it isn’t so far off visually.