If the WP Discourse plugin is activated on your WordPress site, you can write a script that loops through all your site’s users and calls the sync_sso_record function with the SSO params for each user you want to update:
https://github.com/discourse/wp-discourse/blob/main/lib/utilities.php#L78
It is a static function, so you need to call it with:
WPDiscourse\Utilities\Utilities::sync_sso_record($sso_params_array, $wp_user_id)
It has been a while since I tried to do this. Test it with one user before trying to loop through all users on the site.
You could use the Discourse external_name field for the full name. The name you set with the external_name field will become the user’s Name on Discourse. If you need separate first and last name fields, you will have to create custom fields for them on Discourse.
Let me know if you run into any trouble with this. I can try to test it out tomorrow.