Your bank does not have the right to unilaterally change the title on an account. Your responsibility is limited to notifying the survivor and attempting to get him or her to 1`) close the account or 2) agree to the "correction" of the account title and execute a conforming contract.
If an account is interest bearing, you do have the responsibility of filing information returns correctly; i.e. apportioning the interest between the decedent and the survivor. Depending on the circumstances, that may require you to close account A and open account B.