WP Discourse - Relinking Comments After Moving Blog

You would be able to update everything except for the TopicEmbed.embed_url values by republishing the posts from WordPress to Discourse after you change the WordPress domain name. If you take that approach, you’ll need to loop through the TopicEmbeds on your forum and update the embed_url for each one.

A faster, but possibly more dangerous approach would be to use the Discourse remap script. You can find details about using it in this topic: Change the domain name or rename my Discourse?

Running discourse remap domain.com blog.domain.com will rewrite all occurrences of domain.com in your database. Be sure to save a backup of your database before you try it.