Well, it doesn’t quite work that way. Essentially these types of agreements and policies are contracts with your users and customers. If you want these changes to become effective for past users and customers, you will need to get their consent first. That means contacting them by email or through some type of mail delivery service.
And putting a provision like this in your policies will not work either:
It’s a convenient idea, but the law does not work that way.
The courts have held that parties to a contract have no obligation to check a website’s policy on a periodic basis to learn whether they have been changed by the other side.
Why? Because like any other contract, you cannot unilaterally change the terms of the contract without the other party’s consent.