When a plan's or addon's quantity is changed during the middle of a billing period, proration will happen and the subscription will either be charged immediately for the rest of the current billing cycle if the quantity increases or in the case of a downgrade, credits will be applied to the subscription.
Increasing Plan Quantity
Consider a subscription with plan quantity=2 and plan cost is $10/month per unit. If the customer increases the quantity to 3 in the middle of the month, the customer will be charged immediately for $5 as a prorated charge for the rest of the billing period and on the next renewal they will be charged $30.
Decreasing Plan Quantity
For the same subscription, if the customer changes the plan quantity from 2 to 1 in the middle of the month, $5 will be credited as prorated credits. The prorated credits will be applied on the next renewal charge and the customer will only be charged $5. On subsequent renewals, the customer will be charged $10.