If you are a postpaid customer, the plan change happen right away. However, you billing cycle stays the same. (i.e. You have a new plan right away but your billing cycle doesn’t change). Also make sure you are a aware of Koodo data prorating when you change your plan in middle of a billing cycle. More here https://www.koodomobile.com/en/help/proration

If you are a prepaid customer, you get the new plan and your 30 days cycle restart at the time of change.