Automatic key rotation rotates the keys once every year. Automatically rotating keys will have no impact on your ability to decrypt data, as Amazon stores the previous keys for decryption purposes. Additionally, systems that refer to the KMS Key by ARN or Key ID do not need to be updated to point to the new key. There is a small cost of $1 per previous key stored per year, so with automatic key rotation enabled your AWS spend will increase by $1 per key per year, every year.