Upgrade Query: Adding 100K Credits to Pro Plan for One Month

Clay S. - I'm currently on the Pro plan 50K credits a month. I'd like to add another 100K credits just for this month. If I upgrade to the 150K credits a month plan will it charge me the difference between what I was already billed and the new plan cost? Also can I downgrade before the next billing cycle so I am only in that plan for 1 month?