When you opened your savings account you were given account disclosures. There is text in these that describes the interest rate features and what the bank will do when changes are made. First they should have identified the account type as fixed or variable. If it was a fixed rate acount it should have identified the period of time in which the rate would be remain. There still may be a clause allowing them to change that rate after they provide you with a notice of change, at least 30 days in advance. If you don't agree with the change, that period of time is to allow you to "vote with your wallet" and move the account to another product or another bank.
Many accounts today have a variable rate. These accounts require that you be told how often the rate may change. It may say something to the effect of "at our discretion" which means they may change it at will. It may also identify an index and a margin over that amount and it will change based on that.
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