Multiple hostnames for a single site for cut-over phase

There’s a topic for changing host names. Change the domain name or rename your Discourse

If you want multiple names to resolve and for Discourse to redirect to the right one, you’re all set for http. For https you’ll need to get a cert for all domains. I think Set up Let’s Encrypt with multiple domains should help.

If you want to have the site not redirect to itself, you can tweak the nginx to not do that redirect. I’m not sure how to tell you how to do it though.