You will be refunded for the unused time on the current Plan and billed for the new Plan in a single transaction. The renewal date will change to be the current day.
Example. You began Plan A on 1st day of the month. You upgrade from Plan A ($100) to Plan B ($200) in the middle of your billing cycle. (for example the 15th day of the month) You are refunded $50 for the unused time on Plan A. You are charged $200 for Plan B and your billing cycle changes to the 15th day of the month and you have 1 month on the new plan. Total charge $150.