The person moving on does not owe a promise to remain unchanged.

When someone leaves a place, a group or a stage of life, the people staying often want reassurance that nothing essential will shift. It comes out as jokes about not forgetting us, or as hurt when new friends appear. The request is affectionate and impossible: a new life changes a person, which is usually why they went.

If someone close to you is going, tell them you hope it changes them well. Ask about the new place rather than testing whether they miss the old one.