I didn't accept the Yahoo terms, so my emails were not transitioned. All I want to do now is cancel my former email address so that if someone attempts to send mail to it, they'll receive a bounce-back notification.
It seems that after receiving the email from Yahoo, and if you DON'T accept their terms, you are locked out of your account via the webmail interface. However, you can still access emails already received via an email client like Outlook or through migration tools to another provider.
Not sure what happens to emails received in the interim (i.e., after account is locked, but before it is cancelled).