Imagine this, you arrive at home and your partner tells you: from now on we stop eating anything with sugar at home. Your partner then proudly explains that she has put all the sweets, chocolate, and cookies in the trash. 

The idea is certainly, smart and healthy. But you and your sugar tooth are angry!
You're angry because you couldn't prepare yourself for that change! You couldn't eat a last cookie or say goodbye to all these unhealthy chocolate bars you stashed. 

That's basically what happens with a lot of policy changes both at work (1) and with services we use (1). Giving a little bit of heads-up allows people to say goodbye to a habit they had, find an alternative, and maybe also ask you questions about that change.

(1) Imagine being told that you can't bring anymore your bike in the office and this change takes place from now. 
(2) See how Google changed a policy without notice, then had to revert back the change and now promises to let people know in advance next time

This is a first draft of a principle that might end up in a book of the "Service Design Principles" series.