If you upgrade from a paid monthly subscription plan to a higher tier monthly plan, you will be charged immediately for the cost difference between your new plan and your previous plan and all verification volume you've sent through in the current month will be priced according to your new pricing tier. Your subscription will then continue at the start of the next month according to your new plan.
If you upgrade from a paid monthly subscription plan to a higher tier annual plan, you will be charged immediately for the cost difference between your new annual plan and your monthly previous plan. All verification volume you've sent through in the current month will be priced according to your new annual pricing tier, counting as the first month on your new annual plan.
*Please note that verification volumes are applied based on calendar months and subscriptions renew on the first of each month at 12:00AM UTC.