If a financial institution changes the way it determines the rate it will pay on a deposit account (I.e., variable based on Prime versus fixed), is the FI required to provide 30 days notice of the change since it could be adverse to the customer? Or does the fact that it is a business customer not covered under Reg. DD and E mean that no advance notice is required?

It would be governed under your change of terms clause in your business depositor's agreement.