Have you searched for something like ‘old_email’ in the [YourSite]/admin/customize/site_texts search?
On mine, the likely candidates are:
user_notifications.confirm_old_email.text_body_template- (though editing this one gives me ‘access denied’ for some reason?)user_notifications.confirm_old_email_add.text_body_template
But they already have confirm-old-email in there, rather than ‘verify’. Are you up-to-date with your updates?