Hello.

Yes, CSS stylesheet can be switched anytime and it is completely separated from content itself, also page templates can be changed anytime. It is usual approach that you are adding or changing page templates during site´s live, so it will not harm you data.

In general, it depends on what exactly you want to configure before you build your main site, since some data can be bound to page templates (e.g. web part properties or content of some editable web parts), some objects are site-related, so they can be deleted when you delete your testing site, and so on. If you can clarify your goal and describe particular scenario, we can provide more accurate answer. Thank you.

Best Regards,
Radek Macalik