The thesaurus may say that change and transition are synonyms but they aren’t.
Change is the modification of an external situation. It’s meaning is clear. You start a diet. You leave one job for another. You move to a new city. You get married. You have a baby. Each of these is an event. It is a change.
But each of these changes will have their own degree of emotional adjustment, personal adapting, and internal reorientation. The internal reorientation to change is the transition.