This article explains why you may still see old account information in the online Forgot Password flow after changing the security questions, alternate email address or mobile number on file. 

If you recently changed your security questions, alternate email address or mobile number, but you are still able to select the original information as an option through the Forgot Password flow, there is no cause for concern. The option to use your original account information is only available for a limited period of time.

This feature is in place on all Yahoo! accounts to allow the owner of a compromised account to regain access if an unauthorized person has changed their security questions.

For information on ways to protect your information online, please visit the Yahoo! Security Center.