A cancellation discount will kick in when your billing term is completed. This reduces the price of your renewal payments but keeps the same access for your paid plan.
This means monthly plans will discount after the current billing period (less than 30 days from the last successful billing).
The yearly billed monthly plan's discount will kick in when the original subscription term is completed, meaning your plan will have a reduced monthly price after 12 successful billings.
The yearly plan will be discounted once your initial period runs out. It means when your yearly plan renews it will do so at the price shown when you received the cancellation discount.