As a follow up to yesterday’s e-mail, here is an article from Sept. 2015 and Kevin Hunt of the Hartford Courant. It’s still relevant today.
The key point here is that there is no relationship between Frontier and your former ATT Yahoo e-mail. As he says, the account will stay active for as long as ATT supports it. Frontier will provide a separate e-mail. Like me, he recommends an independent e-mail account.
He suggests a method at the end for setting up the Gmail account to check for Yahoo Mail. That is one way to migrate. There are other ways of making the transition.