When you upgrade your account in the middle of a plan period, you only pay the difference between these two plans. Your plan period remains the same. Let's say your plan always starts on the first day of each month. If you change your plan from 5000 to 10000 emails on the 15th day of the month, you will only have 15 more days to use these 10000 emails. Remember that if you have already sent 5,000 emails in this period, you will have 5000 left.