I will add a way to generate embeds for existing posts in a future update. Right now only new posts or posts that have been edited will have their embeds updated. I have not entirely decided how to build the command because it will have to perform a lot of external requests and this could take a long time to complete, in particular if the website doesn't have a queue setup.
Embeds only affect the TextFormatter tags for URL (in practice, this means Autolink [not MediaEmbed, Autovideo or Autoaudio], Markdown links and bbcode URL tag) and IMG (Markdown image tag, bbcode image tag, and Autoimage).
The extension should correctly ignore everything inserted by MediaEmbed, Autovideo and Autoaudio, even if it has images inside of it. It will also not modify images inserted by FoF Upload with Full image or Image preview templates, but it will modify images inserted by FoF Upload with Markdown or bbcode image tags.
Ideally FoF Upload should do this itself for its own images inserted via the custom bbcode tags. My extension is designed for external images, but as long as the image URLs are absolute URLs and can be resolved from the Flarum server it should work fine with images that are hosted on the same server as Flarum as well.
