Thanks!! I was struggling with this and your comment solved my issues. I’m setting up a new WP/Woocommerce/Discourse site, and couldn’t figure out why the comments don’t work. In retrospect it makes perfect sense that existing posts in WP aren’t automatically linked. I was just thinking that by turning on the plugin, the WP comment box itself would always be replaced. It sure would have been easier had there been something to that effect in the plug-in. Currently it says “For Discourse comments to appear on your WordPress site, you must select this setting and enable comments for the WordPress post.” I think it would be easy to add “Existing posts will not have Discourse comment functionality unless they are republished to Discourse.”
I’m also struggling with keeping two comment systems. As mentioned I’m running a Woocommerce site. I’d love to have a single commenting system (Discourse) for UI sake, but not keen on “publishing” hundreds or even thousands of product to Discourse.