When someone opens a new deposit account, the Truth in Savings Act (Regulation DD) requires that you provide them certain account-opening disclosures. These essentially tell them how the account will work, what fees they are likely to incur, what the annual percentage yield will be and if it can change, etc. In a sense, this is your contract with the account holder and if you want to make changes, Regulation DD requires you to give them advance notice of those changes in certain instances. §1030.5(a) states a depository institution shall give advance notice to affected consumers of any change in a term required to be disclosed under §1030.4(b) [aka the required account-opening disclosures] if the change may reduce the annual percentage yield or adversely affect the consumer. The notice shall include the effective date of the change. The notice shall be mailed or delivered at least 30 calendar days before the effective date of the change.
Kevin explains more in the video.