Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Try this: log out, or use the Sign Out option at the upper right, that appears when you click on your first name, next to the 'gear' icon, at the upper right on the Yahoo! Mail page.
Then restart your computer. (If this problem is happening on a computer - if on a phone, or a tablet, restart the phone or tablet.)
Then sign in with the new email and password.
Let me know how that goes, please.