Yes, once a user is created on Discourse through SSO, that user is mapped to Discourse by their SingleSignOn external_id. For the wp-discourse plugin, the external_id is the WordPress user ID. That isn’t affected by changing the email address or username.
If on the Discourse forum you have enabled the settings ‘sso overrides email’, and ‘sso overrides username’, then changes to the username and email address on the WordPress site will also change them on the Discourse site.