Can a bank change after buying out our mortgage demand for us to purchase more flood insurance?

We had refinanced a year ago and it was determined at that time we needed more flood insurance coverage to cover the amount of the loan. We got the additional insurance, then our mortgage was sold. 9 months later the new loan holder wants us to obtain more flood insurance (which our insurance agents assures us in unnnecessary) or they will obtain it themselves and put a lien on us. Is this legal?