I think that most things can be changed retroactively, either in Shell, or the UX.
The only things I would say is think before you start changing default values. If you change a value like “max post length” (default 32000 char) to a larger value, and later change your mind because people start typing “War and Peace” in their posts, then when you reduce it again you’ll have a problem because some of the housekeeping jobs will fall over when they hit the War and Peace posts.
Also, before you go live make sure that you have reviewed and ammended the FAQ to include any rules which are specific to your forum. New users get these as a PM when they sign up, so you might as well get them correct the first time.
Also, If you have a “User Guide” or “Site Announcements” category, be sure to include it in “default categories watching” before you go live. That way, when you go live and you’re playing whack a mole with writing guides and announcing the availability of new features you know that at least the users who haven’t figured out how to change their preferences will see your helpful announcements and guides.