Effective Dating Users on Plans is the best way to ensure historical calculations are kept, but a User is no longer calculated on a given Plan (or at all) in the system. Select the Plan and then specific User you need to update the dates for. Setting the Effective End Date will end all calculations for that User on that Plan.
This is also the best practice for moving Users who stay on the same Plan into a new year. Once an updated Plan structure is built (Calendars, Quotas, Payment Rules) if you push the effective date into the next year the User will be on this Plan for the next year, maintaining continuity.