So it can change in the same way that you can decide right now that your only purposes will be torturing kittens and making giant cheesecakes. It can-as-reachable-node-in-planning do it, not can-as-physical-possibility. So it's possible to build entities with paperclip-maximizing or Friendly goals that will never in fact choose to alter them, just like it's possible for me to trust you won't enslave me into your cheesecake bakery.