I am not planning to do this anytime soon but would like to get an idea in advance.  If I change my domain name will I be able to easily change a setting somewhere and have every thread in the old domain name 301 redirect to the new domain name with the same paths?  Naturally this is to retain SEO.  I own both domain names.

Does anyone have any idea at all?

I'd like to know how to change domain on IPB 4.0 myself.

u can use htaccess domain redirect

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !newdomain.com$ [NC]
RewriteRule ^(.*)$ http://www.newdomain.com/$1 [L,R=301]

and remember to fix License link from "ipb clients section" and in global_config.php

u can also search and replace in database to fix old domain in new (eg for internal link).

maybe a tool like this can be a,good idea: https://interconnectit.com/products/search-and-replace-for-wordpress-databases/

regards