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You mentioned deleting the account from the iPad then setting it up again. When you set it up again, it should prompt you for the login information. Does it accept the username and password ***** that screen?
It sounds like the account was not deleted properly from the iPad then.
Go into Settings then Mail then select the Yahoo account (there might be two screens where you need to select it) then scroll down and press DELETE ACCOUNT.
After that happens, go back into Settings then Mail then accounts (if prompted) then ADD ACCOUNT then add it back.
That is not possible -- If you remove the account from your iPad, it will not bring up old information.
Is the iPad connected to Wi-Fi?
That is likely the issue -- The reason that you see the Yahoo is not available is because you are not connected to Wi-Fi. Also, it is not putting the old password ***** in the box, it is the new password ***** it is not working because it is not connected to Wi-Fi.
Go into Settings then Wi-Fi
Make sure you are connected to your network
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