If you are using the CSV SIS Import then the user_id must always be unique and never change, but the login_id can change.
In my experience, the user_id is the user's sis_id. For us, these ID's correspond to their existing person_id from either the HR (employee) or SIS (student) system, employees are prefixed with an 'E', ensuring we don't have collisions. This means that as long as the user is unique in their system of origin, they are unique in Canvas. The login_id simply provides a link between Canvas and your authentication system, whether that's Canvas, or some other platform. We have had to change authentication systems, and are set to change student logins next year, but nothing changes accept...making your users aware of when the change happens.
Always good practice to first test SIS migrations and changes in beta and test before production.
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